diff --git a/backend/src/cyclone/parsers/parse_835.py b/backend/src/cyclone/parsers/parse_835.py
index 557f5b7..e9974bb 100644
--- a/backend/src/cyclone/parsers/parse_835.py
+++ b/backend/src/cyclone/parsers/parse_835.py
@@ -304,21 +304,20 @@ def _consume_service_payment(segments: list[list[str]], idx: int, line_no: int)
     payment = Decimal(seg[3]) if len(seg) > 3 and seg[3] else Decimal("0")
     units: Decimal | None = None
     unit_type: str | None = None
+    # X12 005010X221A1: SVC04 = Unit or Basis for Measurement Code
+    # (UN, MJ, DA, ...); SVC05 = Service Unit Count.
     if len(seg) > 4 and seg[4]:
-        # In X12 835, SVC04 is "units paid" and SVC05 is the unit-of-measure.
-        # SVC05 may carry a value (UN, MJ, etc.) or be empty.
+        unit_type = seg[4]
+    if len(seg) > 5 and seg[5]:
         try:
-            units = Decimal(seg[4])
+            units = Decimal(seg[5])
         except Exception:
             units = None
-    if len(seg) > 5 and seg[5]:
-        unit_type = seg[5]
-    elif len(seg) > 4 and seg[4]:
-        # Some payers omit SVC05 but still mean "UN" when the units value
-        # is present. Be conservative: only default to UN if no other value
-        # was set above.
-        if unit_type is None:
-            unit_type = "UN"
+    # Some payers omit SVC04 but still mean "UN" when SVC05 carries a
+    # count. Be conservative: only default to UN if no other value was
+    # set above.
+    if unit_type is None and units is not None:
+        unit_type = "UN"
     service_date: date | None = None
     ref_benefit_plan: str | None = None
     adjustments: list[ClaimAdjustment] = []
diff --git a/backend/tests/fixtures/minimal_835.txt b/backend/tests/fixtures/minimal_835.txt
index 223b875..5e30e9a 100644
--- a/backend/tests/fixtures/minimal_835.txt
+++ b/backend/tests/fixtures/minimal_835.txt
@@ -12,9 +12,9 @@ N4*Montrose*CO*814014063~
 LX*1~
 CLP*CLM001*1*100.00*85.00*15.00*MC*CLM001*12*1~
 CAS*CO*45*15.00~
-SVC*HC:S5130:U2*60.00*50.00*7.73*UN*20260602~
+SVC*HC:S5130:U2*60.00*50.00*UN*7.73*20260602~
 DTM*472*20260602~
-SVC*HC:S5130:U2*40.00*35.00*7.73*UN*20260603~
+SVC*HC:S5130:U2*40.00*35.00*UN*7.73*20260603~
 DTM*472*20260603~
 SE*22*991102984~
 GE*1*991102984~
diff --git a/backend/tests/fixtures/unbalanced_835.txt b/backend/tests/fixtures/unbalanced_835.txt
index f247df4..4e52516 100644
--- a/backend/tests/fixtures/unbalanced_835.txt
+++ b/backend/tests/fixtures/unbalanced_835.txt
@@ -12,9 +12,9 @@ N4*Montrose*CO*814014063~
 LX*1~
 CLP*CLM001*1*100.00*85.00*15.00*MC*CLM001*12*1~
 CAS*CO*45*15.00~
-SVC*HC:S5130:U2*60.00*50.00*7.73*UN*20260602~
+SVC*HC:S5130:U2*60.00*50.00*UN*7.73*20260602~
 DTM*472*20260602~
-SVC*HC:S5130:U2*40.00*35.00*7.73*UN*20260603~
+SVC*HC:S5130:U2*40.00*35.00*UN*7.73*20260603~
 DTM*472*20260603~
 SE*22*991102985~
 GE*1*991102985~
diff --git a/backend/tests/test_parse_835.py b/backend/tests/test_parse_835.py
index 3898184..2c78acf 100644
--- a/backend/tests/test_parse_835.py
+++ b/backend/tests/test_parse_835.py
@@ -124,3 +124,57 @@ def test_parse_no_bpr_raises():
     )
     with pytest.raises(CycloneParseError, match="No BPR"):
         parse(text, PayerConfig835.co_medicaid_835())
+
+
+def test_parse_service_payment_units_and_unit_type():
+    """X12 005010X221A1: SVC04 = unit-of-measure, SVC05 = service unit count.
+
+    Bug history: the parser previously read SVC04 as ``units`` and SVC05
+    as ``unit_type`` (with the comment stating the opposite of the spec).
+    On real-world 835s — and the canonical minimal fixture — this left
+    ``units=None`` because SVC04 carries the string "UN" (which fails
+    ``Decimal`` parsing) and SVC05 (a count like "7.73") got assigned to
+    ``unit_type``.
+
+    This test pins down the correct mapping so SP7's line-level matcher
+    can compare units on the SVC side against the claim side.
+    """
+    from decimal import Decimal
+    result = parse(FIXTURE.read_text(), PayerConfig835.co_medicaid_835())
+    c = result.claims[0]
+    s1, s2 = c.service_payments
+    # minimal_835.txt fixture: SVC*HC:S5130:U2*60.00*50.00*UN*7.73*date*
+    assert s1.units == Decimal("7.73"), s1.units
+    assert s1.unit_type == "UN"
+    assert s2.units == Decimal("7.73"), s2.units
+    assert s2.unit_type == "UN"
+
+
+def test_parse_service_payment_units_default_unit_type_to_un():
+    """If SVC04 is empty but SVC05 carries a count, default unit_type to UN.
+
+    Some payers omit the unit-of-measure code when the units are obvious
+    (e.g. always "UN"). The parser should still parse the units and
+    surface a reasonable default for ``unit_type`` so downstream
+    consumers (validation, SP7 line matcher) have a non-None value.
+    """
+    from decimal import Decimal
+    base = FIXTURE.read_text()
+    text = (
+        base
+        .replace(
+            "SVC*HC:S5130:U2*60.00*50.00*UN*7.73*20260602~",
+            "SVC*HC:S5130:U2*60.00*50.00**3*20260602~",
+        )
+        .replace(
+            "SVC*HC:S5130:U2*40.00*35.00*UN*7.73*20260603~",
+            "SVC*HC:S5130:U2*40.00*35.00**4*20260603~",
+        )
+    )
+    result = parse(text, PayerConfig835.co_medicaid_835())
+    s1, s2 = result.claims[0].service_payments
+    assert s1.units == Decimal("3"), s1.units
+    assert s2.units == Decimal("4"), s2.units
+    # Default to UN when SVC04 missing but units present.
+    assert s1.unit_type == "UN", s1.unit_type
+    assert s2.unit_type == "UN", s2.unit_type