Implementation of OFDM-based systems suffers from in-phase and quadrature-phase (IQ) imbalances in the front-end analog processing. The resulting distortion due to IQ imbalances can limit the achievable operating SNR at the receiver, especially when space-time coding is used for enhanced received SNR. In this paper, the effect of IQ imbalances on an OFDM-based system with Alamouti coding is studied and an efficient receiver with compensation for IQ imbalances is then derived. Significant performance improvement is achieved by using the proposed receiver compared to a standard receiver with no compensation for IQ imbalances.