diff --git a/nmrs-gui/src/file_lock.rs b/nmrs-gui/src/file_lock.rs
index 49cef55e..5e54693e 100644
--- a/nmrs-gui/src/file_lock.rs
+++ b/nmrs-gui/src/file_lock.rs
@@ -1,9 +1,18 @@
 use fs2::FileExt;
+use std::fs;
 use std::fs::File;
 
 pub fn acquire_app_lock() -> Result<File, String> {
-    let mut lock_path = dirs::data_local_dir().unwrap_or(std::env::temp_dir());
-    lock_path.push("my_app.lock");
+    // Prefer XDG_RUNTIME_DIR for ephemeral lock files (should be /run/usr/<USR_ID>). Fall back to local data dir.
+    let mut lock_dir = dirs::runtime_dir()
+        .or_else(dirs::data_local_dir)
+        .unwrap_or(std::env::temp_dir());
+
+    lock_dir.push("nmrs");
+
+    fs::create_dir_all(&lock_dir).map_err(|e| format!("Failed to create lock directory: {e}"))?;
+
+    let lock_path = lock_dir.join("app.lock");
 
     let file = File::create(&lock_path).map_err(|e| format!("Failed to create lock file: {e}"))?;
 
@@ -13,3 +22,41 @@ pub fn acquire_app_lock() -> Result<File, String> {
 
     Ok(file)
 }
+
+// Used so each test can be run independently.
+// Without that, with tests being run in parallel, the second test can cause the first one to fail.
+#[cfg(test)]
+use std::sync::Mutex;
+
+#[cfg(test)]
+static TEST_LOCK: Mutex<()> = Mutex::new(());
+
+#[test]
+fn test_lock_file_is_created() {
+    // Test if the lock file is properly created.
+    let _guard = TEST_LOCK.lock().unwrap();
+    let file = acquire_app_lock();
+    assert!(file.is_ok(), "Failed to acquire lock: {:?}", file.err());
+
+    let lock_path = dirs::runtime_dir()
+        .or_else(dirs::data_local_dir)
+        .unwrap_or(std::env::temp_dir())
+        .join("nmrs")
+        .join("app.lock");
+
+    assert!(
+        lock_path.exists(),
+        "Lock file was not created at {:?}",
+        lock_path
+    );
+}
+
+#[test]
+fn test_second_instance_is_rejected() {
+    // Test if acquire_app_lock fails as it should when called twice.
+    let _guard = TEST_LOCK.lock().unwrap();
+    let _first = acquire_app_lock().expect("First lock should succeed");
+    let second = acquire_app_lock();
+    assert!(second.is_err(), "Second instance should have been rejected");
+    assert_eq!(second.unwrap_err(), "Another instance is already running");
+}