For the extremely high sampling rate and high resolution required for multi-Gigabit orthogonal frequency division multiplexing (OFDM) communication systems, time-interleaved analog-to-digital converters (ADCs) are being considered. However, in practice, mismatches such as offset mismatch, gain mismatch and timing mismatch occur between the parallel sub-ADCs. This paper theoretically analyzes the impact of the different mismatches on the performance of the OFDM system. The theoretical results are confirmed by simulations and show that OFDM performance is strongly degraded, even for small mismatches.