This paper analyzes an adaptive modulation scheme for precoded multiple-input multiple-output MIMO systems with minimum-mean-squared-error decision feedback equalizer. The MIMO channel is assumed to be spatially correlated with multipath Rayleigh fading. The zero-padding technique is used to eliminate the inter-block interference. To adjust the constellation size, the signal-to-interference-plus-noise (SINR) ratio at the equalizer output is sent back to the transmitter. In this paper, we derive the probability density function of the SINR for flat fading and frequency selective channels. Then, we present accurate closed-form expressions of the spectral efficiency and the average BER. The derived expressions are compared to simulation results.