This contribution analytically studies the influence of IQ mismatch on the performance of multiple-antenna orthogonal frequency division multiplexing (OFDM) systems based on he zero-IF architecture. First a system model for such a multiple-input multiple-output (MIMO) OFDM system experiencing both transmitter (TX) and receiver (RX) IQ mbalance is derived. This is used to obtain analytical expressions for the probability of error in symbol detection for MIMO OFDM systems in Rayleigh faded multipath channels. The derived results can be used to find matching specifications for the TX- and RX-branches for different multicarrier MIMO systems. It is concluded that in fading channels RX IQ imbalance is on average more destructive than TX IQ imbalance. Additionally, it is concluded that the addition of extra RX antennas is beneficial to reduce the dependence on RX IQ imbalance, but increases at the same time the impact of TX IQ imbalance.