This paper analyzes the performance of chaotic communications in a MIMO system. The robustness of chaos-based communications systems makes Differential Chaos Shift Keying (DCSK) the preferred modulation choice. In order to improve the performance of such a system, the Alamouti space-time code is used for 2 transmit and 2 receive antennas. A new approach for computing the bit-error-rate (BER) performance is provided, and an analytical BER expression is derived. The approach used explores the dynamic properties of chaotic sequences and takes into account the fact that the bit energy varies from one transmitted bit to the next. Simulation results confirm the accuracy of this approach.