The IEEE 802.11e extensions for QoS support in WLAN define the transmission opportunity (TXOP) concept, in order to limit the channel holding times of the contending stations in presence of delay-sensitive traffic. In this paper, we evaluate the use of TXOP for a different purpose: the "temporal fairness" provisioning among stations employing different data rates. We show that the equalization of the channel access times allows each station to obtain its throughput basically (1) proportional to its transmission rate, and (2) independent of the transmitted frame length. This also improves the aggregate throughput of the overall WLAN. For a given TXOP limit, i.e., a granted channel access time, a station is required to fragment its pending frame if the TXOP limit is too short, and is allowed to transmit multiple frames back-to-back in a burst if the TXOP limit is long enough, while different fragmentation and bursting rules are possible. Based on the analytical and simulation results, we demonstrate the advantages of the TXOP operations over the legacy 802.11 DCF, and compare different TXOP managing policies to find the optimal one.