Chemical shifts of the OH proton in supercritical water referenced to the benzene proton have been estimated theoretically using the ah initio molecular orbital (MO) method. The degree of dissociation from hydrogen-bonded water clusters to monomers calculated using the CCSD (T)/6-31+G(d)//MP2 (frozen core)/6-31+G(d) level of theory indicates that supercritical water is comprised of 80% monomer and 20% dimer at the critical point (T-c = 647.1 K, P-c = 22.06 MPa). On the basis of this supercritical water composition, the chemical shift of the OH proton is determined to be -6.19 ppm at the MP2 (frozen core)/6-31+G(d)// MP2 (frozen core)/6-31+G(d) level of theory, which reproduces the recent NMR experimental results well.