Abstract: Several expressions for calculation of δ~(18)O are enumerated in CO_2—H_2O equilibrium. Different δ~(18)O SMOW values for the same exampe are compared. For according with original intention of the expressions, there is a problem when the results are translated to δ~(18)O SMOW values. It is proved that the result obtained is not the δ~(18)0 SMOW value of CO_2 in equilibrium with the original water, but is the δ~(18)O SMOW value of the original water when Craig formula is used according to the convention.