Adaptive bit loading algorithms are proposed for MIMO and MIMO-OFDM systems, thus better exploiting space and frequency diversity. Constant power allocation is assumed over all subchannels while the bit error rate is unconstrained. An efficient bit loading algorithm, which maximizes system bit rate under target mean BER restriction is formulated for a multiple antenna system. A bound on the maximum gap from capacity is introduced as a combination of the constant-power gap and the so-called rate quantization gap.