The hydrolysis of Ba2+ and Ra2+ was studied at 25 degrees C in aqueous mixtures of NaOH and NaClO4 using an ion exchange method and radiotracer and batch techniques. The distribution of Ba-133 and Ra-226 between solid (ion exchange resin) and aqueous (mixture of NaOH and NaClO4) phases was measured via gamma spectrometry and liquid scintillation counting. The total ionic strength was kept constant and the concentration of NaOH in the aqueous phase was varied from 0 (pure NaClO4) to pure NaOH from sample to sample. It was shown that an increase of the Ba2+ or Ra2+ concentration in the aqueous phase with an increase of the NaOH concentration cannot be explained solely by Ba2+ or Ra2+ activity coefficient differences in the NaOH and NaClO4 media (salting out) and that weak BaOH+ and RaOH+ ion pairing occurs in the systems studied. A model for weak ion association was developed and apparent BaOH+ and RaOH+ stability constants were derived assuming the formation of weak aqueous NaOH(aq) ion pairs via non-linear curve fitting. It was demonstrated that systematic uncertainties have a much greater contribution to the NaOH(aq), BaOH+ and RaOH+ stability constant uncertainty budget compared to stochastic uncertainties and a method for estimation of the systematic uncertainties was proposed. The method combines fitting, restricted primitive model computations with surveyed literature data that resulted in a stability constant for NaOH(aq) that ranged from 0 to 1 at ionic strengths below 5 mol.kg (1) (i.e. K-NaOH = 0.5 +/- 0.5, where the uncertainty is a systematic 95% confidence interval). The variation of KNaOH allowed the estimation of the systematic 95% confidence interval in the apparent stability constants of BaOH+ and RaOH+. The specific ion interaction theory was used to extrapolate the derived logarithms of the BaOH+ and RaOH+ apparent stability constants to zero ionic strength (log(10)K = 0.7 +/- 0.2 for both ion pairs) and obtain the relevant ion interaction parameters. It was shown that both the Ba2+ and Ra2+ ions have similar activity coefficients and undergo similar short-range interactions in aqueous NaOH-NaClO4 media. (C) 2018 Elsevier Ltd.