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The paper analyzes the error performance of multiple-input multiple-output (MIMO) systems in frequency-selective Rayleigh fading channels. A zero-forcing (ZF) receiver is applied to separate transmitted data streams for each resolvable multipath component. The decision for each symbol of a transmitted data stream is based on combining L zero-forced multipath components carrying information of the same symbol. Analytical error rate expressions for the system in the absence of intersymbol interference (ISI) and inter-path interference (IPI) are derived first. Then, the effects of ISI and IPI caused by frequency-selective fading on the performance of a MIMO system with direct-sequence spreading applied to each transmitted symbol are studied by simulation. It is found that for a system with N transmit antennas, M receive antennas, and L resolvable multipath components, the ZF receiver combined with a RAKE with maximal ratio combining is equivalent to a 2-dimensional RAKE with a diversity order of (M-N+1)L.