Thin films of organosulfur derivatives adsorbed on Ag(111) electrodes have been investigated by second harmonic generation and by electrochemical techniques. The anisotropic part of the second harmonic signal is generated by the substrate and unaffected by the presence of the film. In contrast, the isotropic signal is strongly influenced by the film. This effect could be caused by the nonlinear polarizability of the films or by the modification of the electronic density at the surface during adsorption.