Conventional analytical methods for the isotopic compositions of brines are fraught with many problems which have not been fully recognized to date. Isotopic fractionation during vacuum distillation, which is involved in the water reduction method for δD determination, is the most serious problem. Both hydrogen and oxygen isotopes in high-CaCl2 and/or -MgCl2 brines can be significantly fractionated during this process. Equilibration of CO2 and brines for δ18O determination was studied in detail; both shaking and acidification are necessary for isotopic equilibration of very high-CaCl2 and/or MgCl2 brines. It is suggested that the published isotopic data of brines are to be re-evaluated with respect to analytical methods employed.