This paper presents an evaluation of the impact of channel time variations on performance of multiuser selection diversity systems employing orthogonal space-time block coding (OSTBC) over MIMO Rayleigh fading channels. A rate-adaptive M-QAM modulation along with the maximum signal-to-noise ratio (SNR) based user selection diversity is considered. Analytical expressions are derived for the average spectral efficiency, average bit error rate (BER), and bit error outage (BEO) of the system. Using the results obtained from the analytical expressions as well as from Monte-Carlo simulations, the impact of channel time variations on the system performance is evaluated.