This paper describes a control mechanism for dynamic selection of Quality of Service (QoS) parameters used in a IEEE 802.11e Wireless Local Area Network (WLAN). The 802.11e specification, still under development, provides a new contention-based access method, called Enhanced Distributed Coordination Function (EDCF). This method uses distinct values for interframe space and for contention window size to establish priorities between Access Categories (ACs). The proposed mechanism selects these values based on the actual network load condition and on the applications delay requirements. Such mechanism improves the new QoS functionality, aiming to promote better channel utilization as well as to keep the desired service levels.