The authors investigate the joint effect of channel estimation and frequency flat transmitter and receiver I/Q imbalance on an Orthogonal Frequency Division Multiplex (OFDM) system. They assume independent fading with identically and independently distributed (i.i.d) mirror carrier channel coefficients. Closed form expressions for symbol error rate and bit error rate of M-QAM modulation are derived. They consider join and separate channel estimation, as well as joint and separate equalization of the mirror carriers. Performance is evaluated by treating the I/Q interference as a non-Gaussian random variable. The results show that a Gaussian approximation of I/Q interference is very good, especially for low order modulations.