The occurring phase transitions in heated M-struvites (i.e. amorphous-amorphous, crystallineamorphous or crystalline-crystalline) are highly dependent on humidity, temperature, heating rate, and on the metal cation in the precursor struvite. From the same isostructural M-struvite precursor, various amorphous and/or transition metal derivates with different pore structures can be obtained. The main phase transition is assumed to be driven by the removal close to five water molecules per formula at around 70-90°C. Mg and Ni-struvite decompose to an amorphous phase at TMg ≈ TNi ≈ 85°C while Co-struvite transforms through a crystallinecrystalline phase transition to Co-dittmarite already at TCo ≈ 72°C. Probably minor amounts of ammonia are also removed in this step. Both amorphous phases of Mg and Ni exhibit a mesoporous network although their evolution and final pore structure differ significantly. This amorphous phase locally resembles that of the Ni-dittmarite structure but is less ordered, at R > 4 Å. Interestingly, similar effects of humidity were observed in iron phosphate minerals . In contrast, Co-struvite decomposed to Co-dittmarite already at room temperature after several days when exposed to air (ESI: Figure , more details in literature ). When heated, this process is accelerated.