A theoretical consideration is carried out of the dangling bond saturation on the Si(I 1 1) surface both in atomic or molecular gas of hydrogen (deuterium) and in hydrogenated (deuterated) electrolyte solution. The isotope effect originating from the difference in the vibrational behavior between D- and H-adatoms is analyzed. The isotope ratios of D- to H-adatom concentrations were calculated for the thermodynamical equilibrium between the Si surface and the relevant gas phase, and for the steady state in the electrolytic systems. The adatom concentrations were obtained from the conditions of equality of the adatom chemical potential to the chemical potential of the atoms in the gas phase or from the conditions of equality of relevant electrochemical reactions rates, respectively. It is shown that the adatom concentrations are affected by the adatom vibration frequencies both in the gas systems and in the electrolytic systems. The numerical evaluations of the isotope ratios were obtained in technologically important temperature ranges from 300 K to 700 K for the gas systems, and from 272 K to 373 K for the liquid systems. For the D or H atomic gas phases, the enhancement of the D- to H-adatom concentrations is shown to occur. The ratio is 3.2 at 300 K and 1.2 at 700 K. On the other hand, the ratio is reduced to 0.8 for the molecular D-2 or H-2 gases, and to about 0.9 for the hydrogenated or deuterated electrolytes. The both ratios decrease slowly with increasing temperature. (C) 2004 Elsevier B.V. All rights reserved.