Abstract Isotropic Raman spectra of aqueous solutions of LiOH, NaOH and KOH at concentrations ranging from high dilution to saturation have been measured and the frequency and width of the OH − stretching band have been analyzed. The dependence of the bandwidth on solute concentration suggests that the OH − vibration undergoes a transition from fast to slow modulation regimes as the solvent concentration decreases below the value of ∼20 water molecules per solute molecule. A correlation between this finding and structural modifications of the H-bond network of the solvent at similar concentrations is envisaged.