Gravimetric, thermogravimetric, and X-ray diffraction analyses, IR spectroscopy, and optical spectroscopy were used in the studies of iso-and non-isothermal transformations of crystalline Pb2PO4NO3.H2O in air at temperature range 20-1000 degrees C. At t>100 degrees C the process proceeds with dehydration of crystal hydrates, accompanied by crystallisation of anhydrous Pb2PO4NO3 which, on heating above 400 degrees C, is decomposed to yield nitrogen dioxide and molar oxygen. The resulting mixture of lead oxide and orthophosphate, which crystallises in monoclinic system, undergoes a transformation at t > 500 degrees C to give an oxo-phosphate Pb4P2O9 melting at 960 degrees C. The paper includes space-time models of development of the thermochemical process involved in the dehydration of initial monohydrate and thermal decomposition of the anhydrous salt formed.