The effect of the solution composition on the luminescence decay time following picosecond laser pulses is examined for CdS single crystals immersed in various aqueous solutions. A digital simulation technique is demonstrated as an efficient tool for the quantitative analysis of luminescence decay curves in the picosecond time domain, using the exact generation function in the simulation. Such analysis yields numerical values for the bulk lifetime, τb, and for the surface recombination velocity, S. A typical value of τb = 2.5 × 10-9 s was found for the CdS crystals employed. The surface recombination velocity evaluated for an etched crystal immersed in distilled water was S ≤ 5 × 103 cm/s. Immersing the etched CdS crystal in a basic sulfide-polysulfide solution increased the surface recombination velocity to a level of 106 cm/s. This indicates a photohole surface capture velocity of S ≥ 106 cm/s at a sulfide-covered CdS surface.