Several feedback-based mechanisms have been proposed to maximize the IEEE 802 .11 MAC protocol efficiency by guaranteeing an optimal time-spreading of users' channel accesses. However, these solutions suffer from a significant unfairness problem in the presence of stations adopting the standard binary exponential backoff algorithm. Generally, a few legacy stations are sufficient to completely starve enhanced stations employing a modified 802 .11 MAC protocol that adapts the backoff to the network contention level. In this paper we propose a credit-based scheme to enforce fairness in heterogeneous networks. Specifically, enhanced stations earn credits when releasing transmission opportunities granted by the standard protocol. The credits are a virtual currency spent by the enhanced stations to reclaim new transmission opportunities. In this paper, we apply our solution to the distributed mechanism for contention control named AOB. Simulation results show the effectiveness of the proposed mechanism in all the network scenarios considered.