B2O3 interacts with hydroxide melts and yields water and a mixture of BO33- and BO45- ions. In this work, the ratio of the concentrations of these two ionic species has been determined at 623 K as a function of the composition of mixtures of sodium and potassium hydroxides in a range extending from 24 to 100 mol% NaOH. The ratio C(BO45-)/C(BO33-) has a maximum value for the 24 mol% NaOH mixture and a minimum value for the 50 mol% mixture. This composition dependence of the stoichiometry of the dissolved boron species was interpreted in relation to changes of the acidobasic properties of hydroxide melts.}