diff --git a/google/google.go b/google/google.go
index 15f8f32..0e5b53d 100644
--- a/google/google.go
+++ b/google/google.go
@@ -188,15 +188,15 @@ func (f *credentialsFile) tokenSource(ctx context.Context, params CredentialsPar
 			return nil, errors.New("missing 'source_credentials' field in credentials")
 		}
 
-		sourceToken, err := f.SourceCredentials.tokenSource(ctx, params)
+		ts, err := f.SourceCredentials.tokenSource(ctx, params)
 		if err != nil {
 			return nil, err
 		}
 		imp := externalaccount.ImpersonateTokenSource{
 			Ctx:       ctx,
-			Url:       f.ServiceAccountImpersonationURL,
+			URL:       f.ServiceAccountImpersonationURL,
 			Scopes:    params.Scopes,
-			Ts:        oauth2.ReuseTokenSource(nil, sourceToken),
+			Ts:        oauth2.ReuseTokenSource(nil, ts),
 			Delegates: f.Delegates,
 		}
 		return oauth2.ReuseTokenSource(nil, imp), nil
diff --git a/google/internal/externalaccount/basecredentials.go b/google/internal/externalaccount/basecredentials.go
index 002b7d8..fd0a02b 100644
--- a/google/internal/externalaccount/basecredentials.go
+++ b/google/internal/externalaccount/basecredentials.go
@@ -126,7 +126,7 @@ func (c *Config) tokenSource(ctx context.Context, tokenURLValidPats []*regexp.Re
 	ts.conf.Scopes = []string{"https://www.googleapis.com/auth/cloud-platform"}
 	imp := ImpersonateTokenSource{
 		Ctx:    ctx,
-		Url:    c.ServiceAccountImpersonationURL,
+		URL:    c.ServiceAccountImpersonationURL,
 		Scopes: scopes,
 		Ts:     oauth2.ReuseTokenSource(nil, ts),
 	}
diff --git a/google/internal/externalaccount/impersonate.go b/google/internal/externalaccount/impersonate.go
index 0d476d1..8251fc8 100644
--- a/google/internal/externalaccount/impersonate.go
+++ b/google/internal/externalaccount/impersonate.go
@@ -29,19 +29,24 @@ type impersonateTokenResponse struct {
 	ExpireTime  string `json:"expireTime"`
 }
 
-// ImpersonateTokenSource uses a source credential, stored in Ts, to request an access token to the provided Url
+// ImpersonateTokenSource uses a source credential, stored in Ts, to request an access token to the provided URL.
 // Scopes can be defined when the access token is requested.
 type ImpersonateTokenSource struct {
-	// execution context
+	// Ctx is the execution context of the impersonation process
+	// used to perform http call to the URL. Required
 	Ctx context.Context
-	// source credential
+	// Ts is the source credential used to generate a token on the
+	// impersonated service account. Required.
 	Ts oauth2.TokenSource
 
-	// impersonation url to request an access token
-	Url string
-	// scopes to include in the access token request
+	// URL is the endpoint to call to generate a token
+	// on behalf the service account. Required.
+	URL string
+	// Scopes that the impersonated credential should have. Required.
 	Scopes []string
-	// Delegates for impersonation to include in the access token request
+	// Delegates are the service account email addresses in a delegation chain.
+	// Each service account must be granted roles/iam.serviceAccountTokenCreator
+	// on the next service account in the chain. Optional.
 	Delegates []string
 }
 
@@ -57,7 +62,7 @@ func (its ImpersonateTokenSource) Token() (*oauth2.Token, error) {
 		return nil, fmt.Errorf("oauth2/google: unable to marshal request: %v", err)
 	}
 	client := oauth2.NewClient(its.Ctx, its.Ts)
-	req, err := http.NewRequest("POST", its.Url, bytes.NewReader(b))
+	req, err := http.NewRequest("POST", its.URL, bytes.NewReader(b))
 	if err != nil {
 		return nil, fmt.Errorf("oauth2/google: unable to create impersonation request: %v", err)
 	}