diff --git a/system_tests/system_tests_sync/test_external_accounts.py b/system_tests/system_tests_sync/test_external_accounts.py
index 59fbd4bef..29355f479 100644
--- a/system_tests/system_tests_sync/test_external_accounts.py
+++ b/system_tests/system_tests_sync/test_external_accounts.py
@@ -42,6 +42,7 @@
 
 import sys
 import google.auth
+from google.auth import _helpers
 from googleapiclient import discovery
 from six.moves import BaseHTTPServer
 from google.oauth2 import service_account
@@ -132,7 +133,6 @@ def get_project_dns(dns_access, credential_data):
     with NamedTemporaryFile() as credfile:
         credfile.write(json.dumps(credential_data).encode("utf-8"))
         credfile.flush()
-        old_credentials = os.environ.get("GOOGLE_APPLICATION_CREDENTIALS")
 
         with patch.dict(os.environ, {"GOOGLE_APPLICATION_CREDENTIALS": credfile.name}):
             # If our setup and credential file are correct,
@@ -150,9 +150,7 @@ def get_xml_value_by_tagname(data, tagname):
 
 # This test makes sure that setting an accesible credential file
 # works to allow access to Google resources.
-def test_file_based_external_account(
-    oidc_credentials, service_account_info, dns_access
-):
+def test_file_based_external_account(oidc_credentials, dns_access):
     with NamedTemporaryFile() as tmpfile:
         tmpfile.write(oidc_credentials.token.encode("utf-8"))
         tmpfile.flush()
@@ -173,10 +171,11 @@ def test_file_based_external_account(
             },
         )
 
+
 # This test makes sure that setting a token lifetime works
 # for service account impersonation.
 def test_file_based_external_account_with_configure_token_lifetime(
-    oidc_credentials, service_account_info, dns_access
+    oidc_credentials, dns_access
 ):
     with NamedTemporaryFile() as tmpfile:
         tmpfile.write(oidc_credentials.token.encode("utf-8"))
@@ -202,6 +201,47 @@ def test_file_based_external_account_with_configure_token_lifetime(
         )
 
 
+def test_configurable_token_lifespan(oidc_credentials, http_request):
+    TOKEN_LIFETIME_SECONDS = 2800
+    BUFFER_SECONDS = 5
+
+    def check_impersonation_expiration():
+        # First, get the default credentials.
+        credentials, _ = google.auth.default(
+            scopes=["https://www.googleapis.com/auth/cloud-platform.read-only"],
+            request=http_request,
+        )
+
+        utcmax = _helpers.utcnow() + datetime.timedelta(seconds=TOKEN_LIFETIME_SECONDS)
+        utcmin = utcmax - datetime.timedelta(seconds=BUFFER_SECONDS)
+        assert utcmin < credentials._impersonated_credentials.expiry <= utcmax
+
+        return True
+
+    with NamedTemporaryFile() as tmpfile:
+        tmpfile.write(oidc_credentials.token.encode("utf-8"))
+        tmpfile.flush()
+
+        assert get_project_dns(
+            check_impersonation_expiration,
+            {
+                "type": "external_account",
+                "audience": _AUDIENCE_OIDC,
+                "subject_token_type": "urn:ietf:params:oauth:token-type:jwt",
+                "token_url": "https://sts.googleapis.com/v1/token",
+                "service_account_impersonation_url": "https://iamcredentials.googleapis.com/v1/projects/-/serviceAccounts/{}:generateAccessToken".format(
+                    oidc_credentials.service_account_email
+                ),
+                "service_account_impersonation": {
+                    "token_lifetime_seconds": TOKEN_LIFETIME_SECONDS,
+                },
+                "credential_source": {
+                    "file": tmpfile.name,
+                },
+            },
+        )
+
+
 # This test makes sure that setting up an http server to provide credentials
 # works to allow access to Google resources.
 def test_url_based_external_account(dns_access, oidc_credentials, service_account_info):
@@ -337,9 +377,7 @@ def test_aws_based_external_account(
 
 # This test makes sure that setting up an executable to provide credentials
 # works to allow access to Google resources.
-def test_pluggable_external_account(
-    oidc_credentials, service_account_info, dns_access
-):
+def test_pluggable_external_account(oidc_credentials, service_account_info, dns_access):
     now = datetime.datetime.now()
     unix_seconds = time.mktime(now.timetuple())
     expiration_time = (unix_seconds + 1 * 60 * 60) * 1000
@@ -354,7 +392,7 @@ def test_pluggable_external_account(
     tmpfile = NamedTemporaryFile(delete=True)
     with open(tmpfile.name, "w") as f:
         f.write("#!/bin/bash\n")
-        f.write("echo \"{}\"\n".format(json.dumps(credential).replace('"', '\\"')))
+        f.write('echo "{}"\n'.format(json.dumps(credential).replace('"', '\\"')))
     tmpfile.file.close()
 
     os.chmod(tmpfile.name, 0o777)