For many years the lowering of the viscosity of water by certain electrolytes has been attributed to the action of their ions in breaking down its special three-dimensional structure. In this paper, the familiar viscosity B-coefficients for the alkali-metal chlorides in aqueous solutions and the related ionic molar contributions to the free energy, enthalpy and entropy of activation, namely Δµ^{°[graphic omitted]}₃, ΔH^°₃ and ΔS^{°[graphic omitted]}₃, are re-examined. Particular attention is paid to the nature of the solvent in the transition state.

The application of the compensation principle is first explored. If compensation between enthalpic and entropic contributions from solute-induced structural changes occurs in both ground- and transition-state solvents, Δµ^{°[graphic omitted]}₃, and hence B cannot by influenced by changes in the structure of the solvent of the type proposed, say, by Frank and Wen. Instead it is suggested that, in aqueous solution, an ion can coordinate more solvent molecules in the more weakly bonded transition-state solvent than in the ground-state solvent. This is particularly important for solutions containing the larger alkali-metal and halide ions, whose enhanced fluidity thus stems not from solvent–solvent bond-breaking in the ground state, but from ion–solvent bond-making in the transition state. Even if the compensation principle is not accepted, this explanation removes structurel changes in the solvent as a unique explanation of the observed trends in B-coefficients.