Phosphate solutions, prepared by mixing concentrated metal chloride solutions with o-phosphoric acid (85%), were used to obtain thin films by spin coating. Using malachite green and Rhodamine 640 dyes as dopant probes, it is possible to follow the drying processes of the films by monitoring the absorption properties of these probe molecules. Changes observed in absorption spectra could be interpreted qualitatively by the evaporation of HCl and H2O from the liquid medium. It is found that during the first stage of drying of the metal phosphate layer both HCl and water evaporate. The evaporation rate depends on the type of dopants. At room temperature the drying process lasts a very long time and the humidity of the film reaches equilibrium with the surrounding atmosphere.