In reactions of aryloxiranes XC 6 H 4(3) CH(O)CH 2 with arenesulfonic acids YC 6 H 4 SO 3 H in a mixture of dioxane with diglyme (1 : 1) at 265 K the nonadditive effects of substituents X and Y were strongly revealed, therefore it was possible to observe experimentally the isoparametricity phenomenon: In the isoparametric point σ X IP = 1.42 with respect to the X substituent constant the rate of the oxirane ring opening did not depend on the structure of Y (ρ Y X = 0).The versatility and features of the mechanisms of oxirane reactions, their unique synthetic possibilities and practical value have attracted the attention of researchers for several decades. The interest in oxirane chemistry is still strong nowadays as show the numerous recent publications (see, e.g., [1]). However although the research on oxiranes is very versatile, the quantitative aspects of their reactions are still poorly understood. The establishment of quantitative laws taking into account the effect of the structure, solvent, temperature, pH of the medium and the other internal and external factors on the rate and regioselectivity of reactions of oxirane substrates with reagents of diverse character is a topical problem. In this respect a special attention should be paid to cross reaction series where occurs the interaction (nonadditivity) of effects of mutually varied factors. These series should be described with the use of polylinear equations with cross terms whose coeffi cients take into account such interactions and therefore their prognostic value signifi cantly grows [2].The goal of this work is the study of the combined effect of substituents X and Y on the reaction rate of substituted aryloxiranes with substituted arenesulfonic acids in a mixture dioxane-bis(2-methoxyethyl) ether (diglyme), 1:1 (Scheme 1). Scheme 1.