In this paper we investigate the performance of IEEE 802.11 ad hoc networks by means of an experimental study. Measurements on IEEE 802.11 ad hoc networks confirm previous simulative results (e.g., TCP connections may actually experience significant throughput unfairness). The analysis of IEEE 802.11b reveals several aspects that are usually neglected in simulative studies. Firstly, since different transmission rates are used for control and data frames, different transmission ranges and carrier-sensing ranges may exist at the same time in the network. In addition, the transmission ranges are in practice much shorter than usually assumed in simulative analysis, and are not constant but highly variable, even in the same session.