diff --git a/src/lib/adif.ts b/src/lib/adif.ts
index b7f1e56..7985995 100644
--- a/src/lib/adif.ts
+++ b/src/lib/adif.ts
@@ -70,6 +70,32 @@ function parseSingleRecord(recordString: string): AdifRecord | null {
   return { fields };
 }
 
+// SSB's ADIF submodes are the sideband, not a distinct operating mode — a phone
+// QSO must stay "SSB" so it still matches the SSB filter and phone statistics.
+const VOICE_SUBMODES = new Set(['USB', 'LSB']);
+
+/**
+ * Collapse an ADIF MODE/SUBMODE pair into the single effective mode Nextlog
+ * stores (the `contacts.mode` column is flat — there is no submode column).
+ *
+ * WSJT-X, JTDX and JS8Call log the meaningful mode as a SUBMODE beneath a
+ * generic parent: FT4/JS8/FST4 under `MFSK`, PSK31 under `PSK`. Keeping only the
+ * parent MODE collapsed all of those into "MFSK"/"PSK", which the mode filter and
+ * mode statistics treat as first-class modes distinct from FT4/JS8/PSK31 — so an
+ * FT4 run imported from WSJT-X silently dropped out of the FT4 filter. We promote
+ * the submode when present, EXCEPT the voice sidebands (USB/LSB) which stay SSB.
+ * Everything is normalized to uppercase to match how modes are stored/exported.
+ */
+export function resolveAdifMode(
+  mode: string | undefined,
+  submode: string | undefined,
+): string {
+  const m = mode?.trim().toUpperCase() ?? '';
+  const sub = submode?.trim().toUpperCase() ?? '';
+  if (sub && !VOICE_SUBMODES.has(sub)) return sub;
+  return m || 'SSB';
+}
+
 /**
  * Insert one parsed ADIF record into the contacts table for the given user +
  * station. Returns a discriminated result describing what happened:
@@ -162,7 +188,7 @@ export async function insertAdifRecord(
     callsign,
     fields.name || null,
     frequency,
-    fields.mode || fields.submode || 'SSB',
+    resolveAdifMode(fields.mode, fields.submode),
     band || null,
     datetime,
     fields.rst_sent || null,
diff --git a/tests/adif-mode.spec.ts b/tests/adif-mode.spec.ts
new file mode 100644
index 0000000..472e823
--- /dev/null
+++ b/tests/adif-mode.spec.ts
@@ -0,0 +1,63 @@
+import { test, expect } from '@playwright/test';
+import { resolveAdifMode, parseAdifRecords } from '@/lib/adif';
+
+// Pure-function tests for the ADIF MODE/SUBMODE resolver. WSJT-X, JTDX and
+// JS8Call log the "digital voice" of a QSO as an ADIF SUBMODE under a generic
+// parent MODE (FT4 under MFSK, JS8 under MFSK, PSK31 under PSK). Storing only
+// the parent MODE collapses every one of those into "MFSK"/"PSK", which the
+// app's mode filter and mode statistics treat as distinct first-class modes —
+// so an FT4 run imported from WSJT-X vanished from the FT4 filter. resolveAdifMode
+// promotes the more-specific submode while keeping phone (SSB) intact.
+
+test.describe('resolveAdifMode', () => {
+  test('promotes a digital submode to the effective mode', () => {
+    // WSJT-X FT4: <MODE:4>MFSK<SUBMODE:3>FT4
+    expect(resolveAdifMode('MFSK', 'FT4')).toBe('FT4');
+    // JS8Call: <MODE:4>MFSK<SUBMODE:3>JS8
+    expect(resolveAdifMode('MFSK', 'JS8')).toBe('JS8');
+    // WSJT-X FST4: <MODE:4>MFSK<SUBMODE:4>FST4
+    expect(resolveAdifMode('MFSK', 'FST4')).toBe('FST4');
+    // fldigi PSK31: <MODE:3>PSK<SUBMODE:5>PSK31
+    expect(resolveAdifMode('PSK', 'PSK31')).toBe('PSK31');
+  });
+
+  test('keeps the parent MODE for voice (USB/LSB) submodes', () => {
+    // SSB phone must stay SSB so it still matches the SSB filter and phone
+    // stats — promoting USB/LSB would split every phone QSO out of SSB.
+    expect(resolveAdifMode('SSB', 'USB')).toBe('SSB');
+    expect(resolveAdifMode('SSB', 'LSB')).toBe('SSB');
+    // Case- and whitespace-insensitive.
+    expect(resolveAdifMode('SSB', ' lsb ')).toBe('SSB');
+  });
+
+  test('falls back to MODE when no submode is present', () => {
+    expect(resolveAdifMode('FT8', undefined)).toBe('FT8');
+    expect(resolveAdifMode('CW', '')).toBe('CW');
+    expect(resolveAdifMode('RTTY', '   ')).toBe('RTTY');
+  });
+
+  test('normalizes to uppercase and trims surrounding whitespace', () => {
+    expect(resolveAdifMode('mfsk', 'ft4')).toBe('FT4');
+    expect(resolveAdifMode(' cw ', undefined)).toBe('CW');
+  });
+
+  test('defaults to SSB when neither mode nor submode is usable', () => {
+    expect(resolveAdifMode(undefined, undefined)).toBe('SSB');
+    expect(resolveAdifMode('', '')).toBe('SSB');
+    expect(resolveAdifMode('   ', '  ')).toBe('SSB');
+  });
+
+  test('resolves the mode of a parsed WSJT-X FT4 record', () => {
+    // A verbatim WSJT-X FT4 log line: the mode lives in SUBMODE, not MODE.
+    const adif =
+      '<call:4>W1AW<gridsquare:4>FN31<mode:4>MFSK<submode:3>FT4' +
+      '<rst_sent:3>+03<rst_rcvd:3>-08<qso_date:8>20240115<time_on:6>123456<eor>';
+
+    const [record] = parseAdifRecords(adif);
+
+    expect(record.fields.mode).toBe('MFSK');
+    expect(record.fields.submode).toBe('FT4');
+    // The importer stores the promoted submode, so it lands in the FT4 filter.
+    expect(resolveAdifMode(record.fields.mode, record.fields.submode)).toBe('FT4');
+  });
+});