The geometrical isotope effect of C-(HO)-O-... type hydrogen bonds was analyzed using the HF/6-31++G** level of multi-component molecular orbital calculations, which take into account the quantum effect of the proton/deuteron. The results of calculation show that the bond lengths of hydrogen-bonded C-H and C-D are 0.4 and 0.5 mAngstrom shorter, respectively, than those of a methane monomer, and that the difference in electronic charge between the hydrogen-bonded proton/deuteron in the C-H(D)O-... are -0.894 and -0.893, respectively. Thus, the geometrical changes and electronic relaxation of a C-(HO)-O-... type hydrogen bond induced by the isotope effect differ from those of typical O-(HO)-O-... and N-(HO)-O-... (C) 2004 Elsevier B.V. All rights reserved.