The proposed IEEE 802.11e draft standard defines new MAC protocols for QoS in wireless networks, mainly EDCF and HCF. EDCF is a contention-based channel access scheme and is part of HCF for infrastructure networks and may be used as a separate coordination function for wireless ad-hoc networks. In this paper, we propose to extend EDCF with a dynamic adaptation algorithm of the maximum contention window (CWmax) for enhanced service differentiation in wireless ad-hoc networks. The purpose of our scheme is to reduce delay and jitter and increase the efficiency of the transmission channel. Priorities between access categories are provisionned by updating the size of the CWmax according to application requirements and channel conditions. The performances of IEEE 802.11e EDCF, enhanced with our adaptation algorithm, are extensively investigated by simulation. Results obtained indicate that the delay and jitter for high priority traffic, i.e., audio traffic, improve and delay decreases by up to 43%. Furthermore, delay for lower priority traffic, e.g., video traffic, remains stable. Throughput in CWmax adaptation, compared to EDCF, is stable in light and medium system load and slightly lower (less than 3%) especially in high loaded system.