diff --git a/src/main/java/org/scribe/model/OAuthRequest.java b/src/main/java/org/scribe/model/OAuthRequest.java
index 0e4041a..a712ae9 100644
--- a/src/main/java/org/scribe/model/OAuthRequest.java
+++ b/src/main/java/org/scribe/model/OAuthRequest.java
@@ -2,18 +2,38 @@ package org.scribe.model;
 
 import java.util.*;
 
+/**
+ * The representation of an OAuth HttpRequest.
+ * 
+ * Adds OAuth-related functionality to the {@link Request}  
+ * 
+ * @author Pablo Fernandez
+ */
 public class OAuthRequest extends Request
 {
-
   private static final String OAUTH_PREFIX = "oauth_";
   private Map<String, String> oauthParameters;
 
+  /**
+   * Default constructor.
+   * 
+   * @param verb Http verb/method
+   * @param url resource URL
+   */
   public OAuthRequest(Verb verb, String url)
   {
     super(verb, url);
     this.oauthParameters = new HashMap<String, String>();
   }
 
+  /**
+   * Adds an OAuth parameter.
+   * 
+   * @param key name of the parameter
+   * @param value value of the parameter
+   * 
+   * @throws IllegalArgumentException if the parameter is not an OAuth parameter
+   */
   public void addOAuthParameter(String key, String value)
   {
     oauthParameters.put(checkKey(key), value);
@@ -21,15 +41,20 @@ public class OAuthRequest extends Request
 
   private String checkKey(String key)
   {
-    if (!key.startsWith(OAUTH_PREFIX))
+    if (!key.startsWith(OAUTH_PREFIX) || key.equals(OAuthConstants.SCOPE))
     {
-      throw new IllegalArgumentException(String.format("OAuth parameters must start with '%s'", OAUTH_PREFIX));
+      throw new IllegalArgumentException(String.format("OAuth parameters must either be %s or start with '%s'", OAuthConstants.SCOPE, OAUTH_PREFIX));
     } else
     {
       return key;
     }
   }
 
+  /**
+   * Returns the {@link Map} containing the key-value pair of parameters.
+   * 
+   * @return parameters as map
+   */
   public Map<String, String> getOauthParameters()
   {
     return oauthParameters;