Abstract The dehydration and condensation reactions during the thermal preparation of tetrametaphosphates (cyclotetraphosphates) of bivalent metals have been followed by thermal analyses under quasi-isothermal, quasi-isobaric conditions. These analyses permitted a more detailed assessment of important factors (reaction temperatures and times, water vapour pressure) that play decisive roles from the technological point of view. A significant favourable influence of increased vapour pressure on the yields of the condensation reactions has been observed. High-temperature differential thermal analysis at various precisely defined conditions of the atmosphere was used to follow the conditions of melting of tetrametaphosphates. Conditions were established for their congruent melting and of their non-congruent melting connected with the formation of higher linear phosphates of vitreous character. Thermal analysis has also been used to evaluate the recrystallization of these polyphosphate glasses with reversible transformation into tetrametaphosphates.