The crystal chemistry associated with the thermal decomposition of hexa-aquomagnesium monoperoxyphthalate is described. Decomposition in an open atmosphere, monitored using powder X-ray diffraction, differential scanning calorimetry, thermogravimetric analysis and infra-red spectroscopy, proceeds, with accompanying water and oxygen evolution, through an amorphous intermediate phase. Upon exposure of the amorphous intermediate to moisture at room temperature a crystalline product is generated which produces a powder X-ray diffraction pattern very similar to that of the peroxy phthalate. Infra-red spectroscopy suggests that decomposition of the persalt may proceed via phthalic anhydride. In a closed environment decomposition occurs only after melting of the peroxyphthalate. The requirement for loss of crystallinity to allow oxygen evolution to take place is discussed.