Transmission opportunity, or TXOP, is a channel control method introduced in the IEEE 802.11e wireless LAN standard for improving channel utilization. In this paper, we propose an analytical model to evaluate the performance of TXOP as a method for achieving efficient burst transmissions in IEEE wireless LANs. We show how the model can be used to estimate the throughput of different access categories as a function of the TXOP limit value, and how to calculate the total throughput achieved under basic-access and RTS/CTS access modes. We also show that improved service differentiation can be achieved by using a novel scheme based on TXOP thresholds. Our model demonstrates how the combined used use of TXOP with other MAC parameters can lead to higher aggregate throughput and improved service differentiation.