diff --git a/src/main/java/org/polystat/Program.java b/src/main/java/org/polystat/Program.java
index a181e209..8045d69f 100644
--- a/src/main/java/org/polystat/Program.java
+++ b/src/main/java/org/polystat/Program.java
@@ -26,7 +26,6 @@
 
 import com.jcabi.xml.XML;
 import com.jcabi.xml.XMLDocument;
-import java.nio.file.Files;
 import java.nio.file.Path;
 import java.util.List;
 import org.cactoos.Func;
@@ -74,10 +73,14 @@ public XML apply(final String locator) throws Exception {
         final String[] parts = locator.split("\\.");
         final String name = parts[1];
         final Path xml = this.temp.resolve(String.format("%s.xml", name));
-        if (!Files.exists(xml)) {
+        final Path src = this.sources.resolve(String.format("%s.eo", name));
+        if (
+            !xml.toFile().exists()
+                || xml.toFile().lastModified() < src.toFile().lastModified()
+        ) {
             new Syntax(
                 name,
-                new InputOf(this.sources.resolve(String.format("%s.eo", name))),
+                new InputOf(src),
                 new OutputTo(xml)
             ).parse();
             new Xsline(
diff --git a/src/test/java/org/polystat/ProgramTest.java b/src/test/java/org/polystat/ProgramTest.java
index eef23669..72dcbd05 100644
--- a/src/test/java/org/polystat/ProgramTest.java
+++ b/src/test/java/org/polystat/ProgramTest.java
@@ -24,13 +24,15 @@
 package org.polystat;
 
 import com.jcabi.xml.XML;
-import java.nio.charset.StandardCharsets;
-import java.nio.file.Files;
 import java.nio.file.Path;
+import org.cactoos.Text;
 import org.cactoos.io.ResourceOf;
+import org.cactoos.io.TeeInput;
+import org.cactoos.scalar.LengthOf;
 import org.cactoos.text.TextOf;
 import org.hamcrest.MatcherAssert;
 import org.hamcrest.Matchers;
+import org.junit.jupiter.api.Assertions;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.io.TempDir;
 
@@ -42,14 +44,44 @@
 final class ProgramTest {
 
     @Test
-    void interpretOneEolangProgram(@TempDir final Path sources,
-        @TempDir final Path temp) throws Exception {
-        Files.write(
-            sources.resolve("foo.eo"),
-            new TextOf(
-                new ResourceOf("org/polystat/tests/div-by-zero.eo")
-            ).asString().getBytes(StandardCharsets.UTF_8)
+    void interpretOneEolangProgram(
+        @TempDir final Path sources,
+        @TempDir final Path temp
+    ) throws Exception {
+        this.writeSourceFile(sources);
+        this.assertOutput(sources, temp);
+    }
+
+    @Test
+    void recompilesIfExpired(
+        @TempDir final Path sources,
+        @TempDir final Path temp
+    ) throws Exception {
+        this.writeSourceFile(sources);
+        final Path xml = temp.resolve("foo.xml");
+        this.writeFile(new TextOf("INVALID"), xml);
+        Assertions.assertTrue(
+            xml.toFile().setLastModified(0L)
         );
+        this.assertOutput(sources, temp);
+    }
+
+    /**
+     * Write to file.
+     * @param data Data.
+     * @param file File.
+     */
+    private void writeFile(final Text data, final Path file) {
+        new LengthOf(new TeeInput(data, file)).longValue();
+    }
+
+    /**
+     * Run program & check output.
+     * @param sources Source dir.
+     * @param temp Temp dir.
+     * @throws Exception If program fails.
+     */
+    private void assertOutput(final Path sources, final Path temp) throws Exception {
         final Program program = new Program(sources, temp);
         final XML foo = program.apply("\\Phi.foo");
         MatcherAssert.assertThat(
@@ -58,4 +90,16 @@ void interpretOneEolangProgram(@TempDir final Path sources,
         );
     }
 
+    /**
+     * Write source code.
+     * @param dir Output dir.
+     */
+    private void writeSourceFile(final Path dir) {
+        this.writeFile(
+            new TextOf(
+                new ResourceOf("org/polystat/tests/div-by-zero.eo")
+            ),
+            dir.resolve("foo.eo")
+        );
+    }
 }