Wet photoelectrochemical etching of GaAs was investigated with the use of a numerical model that calculated electron, hole, and potential distributions in a selectively illuminated semiconductor. The Galerkin finite element method was used to solve the Poisson equation for the potential and the species transport equations for holes, and electrons in two dimensions. The transport equations included generation, recombination, diffusion, and migration terms. A technique was developed for examining the sensitivity of the distribution of holes, electrons, and the potential to the parameters of the system including the surface reaction rate constant, diffusion coefficients, doping concentration, light intensity, wavelength, and the recombination length. The sensitivity analysis technique facilitated identification of parameters for which the behavior of this complex, nonlinearly coupled system is most sensitive.