Display omitted ► Two models for the estimation of stability of lanthanide complexes were developed. ► Models are suitable at low and high mass fractions of dioxane in water. ► Models enable simultaneous estimation of K1 in different water–dioxane mixtures. ► Models allow simultaneous estimation of K1 of complexes of different metals. We developed two models for the estimation of stability constants in dioxane–water mixtures of mono-complexes of lanthanides (La3+, Ce3+, Pr3+, and Nd3+) with carboxylic acids (ethanoic, propanoic, and butanoic). The first, linear model proved suitable for the prediction of logK1 at low mass fractions of dioxane, w=0–20% (r=0.981, S.E.=0.06), whereas the second, quadratic model, yielded better results at higher fractions, w=40–60% (r=0.980, S.E.=0.10).