Multiple-input multiple-output (MIMO) wireless communication systems provide high capacity due to the plurality of modes available in the channel. In this paper, we investigate the achievable capacity when using such systems with body area networks (BANs). Intensive measurements in the 2.45 GHz band have been conducted on the three belt-head, belt-wrist and belt-chest on-body channels. Based on these measurements, the slow and fast fading parameters as well as the correlation matrices are extracted and two models are generated for each on-body channel. The second model, as we show, results in a better agreement with the measured channel. The maximum achievable capacity is then evaluated when using the classic uniform power and the waterfilling allocation schemes.