Heteropoly compounds can form several different crystallohydrates, but their stability depends on the ambient temperature and relative humidity. Therefore we have investigated 12-tunastophosphoric acid (WPA) alkaline earth salts under constant conditions - at relative humidity (35%) and room temperature (20degreesC). Under these conditions, hydration degree and phase transformations have been Studied by thermal analyses (TGA, DTA), IR spectroscopy and conductometric measurements. The obtained results proved the influence of the cations nature on the dehydration process.