Molar excess enthalpy data at 308.15 K for 1 M binary solutions of MX + NY (or NX), MY + NX (or NY), M(or N)X + M(or N)Y (where M is Na+, N is K+, X is Cl-, Y is Br-) electrolytes in 1,4-dioxane (D) + water (x(D) = 0.0142, dielectric constant 70 at 308.15 K) solvent (S) system have been determined using an LKB flow microcalorimeter. The data, which differ appreciably from the corresponding data in water and which depend, unlike those in water, on the nature of the common cation and common anion, do not support Young’s cross square rule; this failure has been attributed to the dependence of the hydration of the ions of the MX + NX or MX + NY, etc., 1:1 electrolyte mixtures in S on the nature of (i) the common cation and common anion and (ii) the mixture of anions and of cations. The H(E) data of the present systems have been attributed to both the coulombic interactions and the asymmetric effects, and this has been substantiated by an analysis of the data in terms of Friedman’s cluster theory. The importance of the virial theory of Pitzer in the determination of the relative magnitudes of the coulombic and asymmetric interactions on H(E) data, is also highlighted.