Providing differentiated Quality of Service (QoS) levels is an important challenge for wireless ad hoc networks and wireless LANs when applications have diverse performance requirements. The IEEE 802.11e MAC protocol can provide a Dynamic MAC by assigning different AIFSs, contention window expansion factors (PFs), and (CWmin, CWmax) pairs for different classes and can provide a Static MAC by adjusting the durations of AIFSs based on priority levels [Aad01]. In this paper, we propose a novel and efficient service differentiation mechanism via the C-MAC. In our protocol, each node will change its backoff counter based on both its own packet's priority level and the priority level of the transmitted packet. The simulation results indicate that the Static MAC provides a service differentiation at the expense of significant goodput degradation when the amount of high priority class traffic is low. On the other hand, the Dynamic MAC fails to prevent low priority classes accessing the channel resulting in significant high priority class goodput degradation when the network load is high. However, our mechanism always provides an efficient service differentiation mechanism and high goodput with a small goodput degradation. 2006 International Federation for Information Processing.