The diode diameter dependence of current density in mesa p-n junctions was used to measure surface recombination velocities (S) for both InGaP and AlGaAs. For InGaP, S values of 4-5 X 10(4) cm s-1 were obtained for both wet- and dry-etched mesas, and the surface was relatively insensitive to changes resulting from annealing or plasma exposure. Surface passivation by (NH4)2Sx treatment reduced the recombination velocity by a factor of 2. By contrast, AlGaAs displayed a strong sensitivity to the type of processing steps used in photonic and electronic device fabrication, with values of S as high as 9 x 10(5) cm s-1 after low temperature annealing, and as low as 3.7 x 10(4) cm s-1 after sulphide passivation.