diff --git a/include/tinysd/sdio.h b/include/tinysd/sdio.h
index 29c2337..13aca5d 100644
--- a/include/tinysd/sdio.h
+++ b/include/tinysd/sdio.h
@@ -79,6 +79,10 @@
 #define TSD_SDIO_BUS_WIDTH_4BIT              2
 #define TSD_SDIO_BUS_WIDTH_8BIT              3
 
+/** Card capability register fields. */
+#define TSD_SDIO_CAPABILITY_4BLS             TSD_BIT(7)
+#define TSD_SDIO_CAPABILITY_LSC              TSD_BIT(6)
+
 /** Bus speed select register fields. */
 #define TSD_SDIO_SPEED_EHS                   TSD_BIT(1)
 #define TSD_SDIO_SPEED_SHS                   TSD_BIT(0)
@@ -149,7 +153,7 @@ int tsd_sdio_identify(struct tsd_host *host, struct tsd_sdio_cis *cis);
  *
  * @param card the card
  * @param host the host the card sits on, started by the caller
- * @param width the bus width, 1 or 4
+ * @param width the bus width the host can run, 1 or 4
  * @param clock_khz the highest clock in kHz, held lower when the card cannot run it
  * @return 0 or a negative tsd_error
  */
diff --git a/src/sdio_init.c b/src/sdio_init.c
index 5544237..dac30f5 100644
--- a/src/sdio_init.c
+++ b/src/sdio_init.c
@@ -227,13 +227,28 @@ static int enumerate_card(struct tsd_sdio *card, struct tsd_host *host)
 // Switch the card and then the host between 1-bit and 4-bit mode
 static int set_bus_width(struct tsd_sdio *card, uint8_t width)
 {
+  // Read the card capability register
+  uint8_t capability;
+  int ret = tsd_sdio_read_f0(card, TSD_SDIO_CCCR_CARD_CAPABILITY, &capability);
+  if (ret < 0)
+    return ret;
+
+  // Keep a low speed card on one data line unless it supports four
+  if ((capability & TSD_SDIO_CAPABILITY_LSC) && !(capability & TSD_SDIO_CAPABILITY_4BLS))
+    width = 1;
+
+  // Pick the width field's value
+  uint8_t field = TSD_SDIO_BUS_WIDTH_1BIT;
+  if (width == 4)
+    field = TSD_SDIO_BUS_WIDTH_4BIT;
+
   // Read the bus interface control register and set its width field
   uint8_t interface;
-  int ret = tsd_sdio_read_f0(card, TSD_SDIO_CCCR_BUS_INTERFACE_CONTROL, &interface);
+  ret = tsd_sdio_read_f0(card, TSD_SDIO_CCCR_BUS_INTERFACE_CONTROL, &interface);
   if (ret < 0)
     return ret;
-  interface = (interface & ~TSD_SDIO_BUS_WIDTH) |
-              TSD_FIELD_PREP(TSD_SDIO_BUS_WIDTH, width == 4 ? TSD_SDIO_BUS_WIDTH_4BIT : TSD_SDIO_BUS_WIDTH_1BIT);
+  interface &= ~TSD_SDIO_BUS_WIDTH;
+  interface |= TSD_FIELD_PREP(TSD_SDIO_BUS_WIDTH, field);
 
   // Switch the card first, so it is listening on four lines when the host starts driving them
   ret = tsd_sdio_write_f0(card, TSD_SDIO_CCCR_BUS_INTERFACE_CONTROL, interface);
diff --git a/test/test_sdio.c b/test/test_sdio.c
index cad0b38..0102f18 100644
--- a/test/test_sdio.c
+++ b/test/test_sdio.c
@@ -183,6 +183,20 @@ static void test_low_speed(void)
   TEST_ASSERT_EQUAL_UINT32(400, host.clock_khz);
 }
 
+// Test that a low speed card stays on one data line unless it supports four
+static void test_low_speed_width(void)
+{
+  uint8_t *interface = &host.f0[TSD_SDIO_CCCR_BUS_INTERFACE_CONTROL];
+
+  host.f0[TSD_SDIO_CCCR_CARD_CAPABILITY] = TSD_SDIO_CAPABILITY_LSC;
+  TEST_ASSERT_EQUAL(0, tsd_sdio_init(&card, &host.base, 4, 400));
+  TEST_ASSERT_EQUAL(TSD_SDIO_BUS_WIDTH_1BIT, TSD_FIELD_GET(TSD_SDIO_BUS_WIDTH, *interface));
+
+  host.f0[TSD_SDIO_CCCR_CARD_CAPABILITY] = TSD_SDIO_CAPABILITY_LSC | TSD_SDIO_CAPABILITY_4BLS;
+  TEST_ASSERT_EQUAL(0, tsd_sdio_init(&card, &host.base, 4, 400));
+  TEST_ASSERT_EQUAL(TSD_SDIO_BUS_WIDTH_4BIT, TSD_FIELD_GET(TSD_SDIO_BUS_WIDTH, *interface));
+}
+
 // Test that a block size past a function's limit is refused
 static void test_block_size_limit(void)
 {
@@ -214,6 +228,7 @@ int main(int argc, char **argv)
   RUN_TEST(test_response_flags);
   RUN_TEST(test_high_speed);
   RUN_TEST(test_low_speed);
+  RUN_TEST(test_low_speed_width);
   RUN_TEST(test_block_size_limit);
   RUN_TEST(test_byte_mode_limit);