An analysis of the ir spectra of HF–H2O mixtures containing from 10 to 97% of HF has been used to determine the evolution of the ionic species of HF. The results indicate that water and the ions H3O+F−, H3O+, and F− are present in the solutions containing less than 46% w/w of HF. In the solutions containing at least 50% w/w of HF, bands characteristic of the ion HF2− start appearing and their intensity increases with the concentration of HF, up to a concentration of 90%. As well, polymers of (HF)n have been detected in the solutions containing the highest concentration of HF. These results can be used to explain the electrochemical behaviour of solutions of HF–H2O.