Density functional theory was used to calculate the global hardness and the local (condensed) softness of the free and monosolvated hydrosulfide-ion HS- in order to carry out an analysis of the solvation effect on the chemical reactivity of the ion and its interaction with CaAs surface. The solvents were water and different single-based alcohols. Sulfur atom in free and hydrated HS- ion is rather reactive toward both nucleophiles and electrophiles while water attached to the latter ion is inert. In contrast, sulfur atom in the ion solvated by an alcohol molecule is strongly nucleophilic whereas the solvent is strongly electrophilic. Besides, the nucleophility of sulfur atom in such an ion is the greater, the softer molecule solvates the ion. Accordingly, the adsorption of hydrated and solvated by alcohol molecule ions onto GaAs surface will proceed by fundamentally different mechanisms. This explains the previous experimental findings of the profound effect of the adsorbate solvation on the resulting semiconductor surface electronic structure.