The GaAs etching by the hydrogen peroxide-succinic acid mixture in an ammoniacal medium was studied, and the activation energy of the overall reaction was determined. It was shown that diffusion influences the etching sections and the roughness of the etched surface. Optimal conditions for etching (pH, temperature, agitation, thiourea concentration) were established by observing the samples' surfaces using optical microscopy and atomic force microscopy (AFM). The addition of thiourea to the etching bath leads to a very significant reduction in the surface's roughness. The increase in the intensity of the photoluminescence at 4 K of the etched GaAs samples in the bath containing thiourea could indicate a definite reduction in nonradiative emissions, and therefore a passivation of the surface.