Several studies in literature have investigated the performance of the proposed IEEE 802.11E standard for QOS differentiation in WLAN, but most of them are limited both with respect to the range of the parameter settings and the considered traffic scenarios. The aim of the present study is twofold. First, we systematically investigate the differentiating capabilities of QOS mechanisms. Second, we investigate how well the QOS mechanisms are able to support different types of services under realistic traffic conditions. In particular, we investigate flow-level performance characteristics (e.g., file transfer times) in the situation that the number of active stations varies dynamically in time.