Photoluminescence from GaAs/electrolyte junctions can provide detailed information concerning the kinetics of surface recombination. Here we present the results of a study of picosecond laser excited photoluminescence from the n-GaAs/Na2S electrolyte junction. We have developed a model for the dependence of the photoluminescence efficiency (PLE) on the surface minority carrier trapping rate following photoexcitation with a short light pulse. The model indicates that the functional dependence of the PLE on the surface minority carrier trapping rate is identical to that under CW excitation. The PLE from the n-GaAs/Na2S junction is shown to depend strongly on the excitation intensity, which we attribute to surface trap state filling. We have used our model to measure the surface minority trapping velocity as a function of excitation power at the flat band potential. From these measurements we conclude that the residence time of minority carriers within the trap states is on the order of microseconds, which is 3 orders of magnitude longer than the bulk minority carrier lifetime.