The effect on the Schottky barrier height of H-monolayer passivation of the Si(111) surface is explored. In our calculations we have analyzed K/Si(111) interfaces (theta = 1/3 ML) with and without a H interlayer. Our results show that the effect of passivation is to reduce the Schottky barrier height, phi(bn), by 0.23 eV. Comparison is made with previous results on passivated GaAs(110) surfaces.