diff --git a/spring-cloud-sleuth-core/src/main/java/org/springframework/cloud/sleuth/Span.java b/spring-cloud-sleuth-core/src/main/java/org/springframework/cloud/sleuth/Span.java
index e44e6d44a..394ce657d 100644
--- a/spring-cloud-sleuth-core/src/main/java/org/springframework/cloud/sleuth/Span.java
+++ b/spring-cloud-sleuth-core/src/main/java/org/springframework/cloud/sleuth/Span.java
@@ -414,12 +414,34 @@ public class Span {
 	}
 
 	/**
-	 * Represents given long id as hex string
+	 * Represents given long id as 16-character lower-hex string
 	 */
 	public static String idToHex(long id) {
-		return Long.toHexString(id);
+		char[] data = new char[16];
+		writeHexLong(data, 0, id);
+		return new String(data);
 	}
 
+	/** Inspired by {@code okio.Buffer.writeLong} */
+	static void writeHexLong(char[] data, int pos, long v) {
+		writeHexByte(data, pos + 0,  (byte) ((v >>> 56L) & 0xff));
+		writeHexByte(data, pos + 2,  (byte) ((v >>> 48L) & 0xff));
+		writeHexByte(data, pos + 4,  (byte) ((v >>> 40L) & 0xff));
+		writeHexByte(data, pos + 6,  (byte) ((v >>> 32L) & 0xff));
+		writeHexByte(data, pos + 8,  (byte) ((v >>> 24L) & 0xff));
+		writeHexByte(data, pos + 10, (byte) ((v >>> 16L) & 0xff));
+		writeHexByte(data, pos + 12, (byte) ((v >>> 8L) & 0xff));
+		writeHexByte(data, pos + 14, (byte)  (v & 0xff));
+	}
+
+	static void writeHexByte(char[] data, int pos, byte b) {
+		data[pos + 0] = HEX_DIGITS[(b >> 4) & 0xf];
+		data[pos + 1] = HEX_DIGITS[b & 0xf];
+	}
+
+	static final char[] HEX_DIGITS =
+			{'0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f'};
+
 	/**
 	 * Parses a 1 to 32 character lower-hex string with no prefix into an unsigned long, tossing any
 	 * bits higher than 64.
diff --git a/spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/SpanTests.java b/spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/SpanTests.java
index bed930500..e39c07c8c 100644
--- a/spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/SpanTests.java
+++ b/spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/SpanTests.java
@@ -35,12 +35,12 @@ import static org.assertj.core.api.BDDAssertions.then;
 public class SpanTests {
 
 	@Test
-	public void should_convert_long_to_hex_string() throws Exception {
+	public void should_convert_long_to_16_character_hex_string() throws Exception {
 		long someLong = 123123L;
 
 		String hexString = Span.idToHex(someLong);
 
-		then(hexString).isEqualTo("1e0f3");
+		then(hexString).isEqualTo("000000000001e0f3");
 	}
 
 	@Test
diff --git a/spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/instrument/zuul/TraceRestClientRibbonCommandFactoryTest.java b/spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/instrument/zuul/TraceRestClientRibbonCommandFactoryTest.java
index f85932401..a87ce8f78 100644
--- a/spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/instrument/zuul/TraceRestClientRibbonCommandFactoryTest.java
+++ b/spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/instrument/zuul/TraceRestClientRibbonCommandFactoryTest.java
@@ -93,13 +93,13 @@ public class TraceRestClientRibbonCommandFactoryTest {
 
 		HttpRequest httpRequest = builder.build();
 		then(httpRequest.getHttpHeaders().getFirstValue(Span.SPAN_ID_NAME))
-				.isEqualTo("1");
+				.isEqualTo("0000000000000001");
 		then(httpRequest.getHttpHeaders().getFirstValue(Span.TRACE_ID_NAME))
-				.isEqualTo("2");
+				.isEqualTo("0000000000000002");
 		then(httpRequest.getHttpHeaders().getFirstValue(Span.SPAN_NAME_NAME))
 				.isEqualTo("name");
 		then(httpRequest.getHttpHeaders().getFirstValue(Span.PARENT_ID_NAME))
-				.isEqualTo("3");
+				.isEqualTo("0000000000000003");
 		then(httpRequest.getHttpHeaders().getFirstValue(Span.PROCESS_ID_NAME))
 				.isEqualTo("processId");
 	}