The inhibiting (in acidic media) and catalytic (in neutral media) effects of thiourea (TU) on H-2 evolution at a silver electrode has been studied by linear sweep voltammetry. The influence of the nature and concentration of anions present in TU solutions on this process has been determined. The results of electrochemical measurements are discussed in terms of the coadsorption of anions with TU and the different orientations of TU molecules with respect to the electrode surface. This orientation, as determined from surface-enhanced Raman spectra, depends on the TU concentration and the type of anion. It is postulated that in neutral solutions NH2 groups H-bonded to H2O or H3O+ molecules mediate in proton discharge. In acidic media an interaction of NH bonds with anions is responsible for the inhibiting effect of TU on H-2 evolution.