Passive RFID systems used for inventory management and asset tracking typically utilize contention-based MAC protocols, such as the standard C1G2 protocol. Although the C1G2 protocol has the advantage that it is easy to implement, it suffers from unfairness and relatively low throughput when the number of tags in the network increases. This paper proposes a token-based MAC protocol called Token-MAC for passive RFID systems, which aims a) to provide a fair chance for tags in the network to access the medium without requiring synchronization of the tags, b) to increase the overall throughput, i.e., the tag rate, and c) to enable a high number of tags to be read under limited tag read time availability, which is an especially important challenge for mobile applications. We implement Token-MAC as well as C1G2 and a TDMA-based protocol using Intel WISP passive RFID tags and perform experiments. Additionally, based on our experimental results, we develop energy harvesting and communication models for tags that we then use in simulations of the three protocols. Our experimental and simulation results all show that Token-MAC can achieve a higher tag rate and better fairness than C1G2, and it can provide better performance over a longer range compared with the TDMA-based protocol. It is also shown that Token-MAC achieves much lower tag detection delay, especially for high numbers of tags. Token-MAC is, therefore, a promising solution for passive RFID systems.