The IEEE 802.11e medium access control (MAC) standard provides distributed service differentiation or Quality-of-Service (QoS) by employing a priority system. In 802.11e networks, network traffic is classified into different priorities or access categories (ACs). Nodes maintain separate queues for each AC and packets at the head-of-line (HOL) of each queue contend for channel access using AC-specific parameters. Such a mechanism allows the provision of differentiated QoS where high priority, performance sensitive traffic such as voice and video applications will enjoy less delay, greater throughput and smaller loss, compared to low priority traffic (e. g. file transfer). The standard implicitly assumes that nodes are honest and will truthfully classify incoming traffic into its appropriate AC. However, in the absence of any additional mechanism, selfish users can gain enhanced performance by selectively classifying low priority traffic as high priority, potentially destroying the QoS capability of the system. The focus of this paper is to understand the impact of selfish users on network performance and design MAC layer mechanisms which ensure that selfish users do not sabotage network operation. More specifically, we consider an infrastructure based slotted Aloha like MAC which resembles the 802.11e in many essential respects. We chose to employ ALOHA instead of using the 802.11e model directly because the resulting system is more amenable to analysis. We believe that the insights obtained by using our simple MAC are directly applicable to the 802.11e scenario and can inform future standard evolution. In our system, nodes communicate with each other through