Properties of the mixed adsorption layer of n-butanol-thiourea on the mercury/1 M NaClO4 solution interface were described using their adsorption parameters as well as kinetic ones for the piloting process of Zn(II) ion reduction. Butanol is an inhibitor of this process but thiourea accelerates it according to the cap-pair rule. Additivity of thiourea and butanol adsorption was found. For the concentration ratio of butanol : thiourea almost-equal-to 12 the compensation of inhibiting and accelerating effects in a wide range of concentrations was observed. Formation of mixed adsorption layers was experimentally confirmed.