It has been assumed that the structure of binary solution may change depending on the molar ratio of solute to solvent and refer to the presence of some critical concentration where solute-solvent interactions change. Spectroscopic investigation on aqueous solutions of propylene carbonate and alkali halides have really shown presence of critical concentrations. Furthermore, the latter critical concentration of aqueous alkali halide solutions has revealed concern with the reactivity of iodide ions which have been added to the solutions. The latter finding will be the first observation of the concern between structure and reactivity of aqueous solutions.