Phosphates can be thermally dehydrated to polyphosphates or metaphosphates. This is an important reaction in the preparation of glasses, catalysts and flame retardants. In this study, we have monitored the dehydration process of potassium and sodium dihydrogen phosphates, using in situ Raman spectroscopy, thermal analysis, X-ray powder diffraction and infrared spectroscopy. We obtained a : good correlation between the thermal data and the structural phase transitions observed during the reactions, using Raman and infrared spectroscopy. Deuteration of the samples assisted us in assigning some of the vibrational bands.