In the case of single-element struvites, all three considered compounds exhibited different thermally-induced phase transitions despite sharing the same crystal structure. Importantly Ni-struvite turned out to convert into a stable amorphous phase, and Co-struvite turned into crystalline Co-dittmarite. The question arose how would mixed NixCo1-x-struvites evolve under similar thermal conditions in particular that the mixed transition metal struvites had different thermal profiles (Figure , ESI: Figure ) from their mono-cationic analogues. In the first step, we confirmed that for a broad range of compositions we indeed synthesized phase-pure NixCo1-x-struvites with x = 0-1 (ESI: Figure ). Based on EDS mappings, a complete solid solution of NixCo1-x-struvite with homogenous distribution of both metals regardless of the Ni# could be identified at least on the sub-µm-scale (ESI: Figure , Table ). Upon heating, the synthesized NixCo1-x struvite transformed to either amorphous or crystalline phases after the thermal treatment depending on the Ni# (Eq. Eq. [1], Figure ). Ni-rich NixCo1-x-struvites above Ni# >0.5 decomposed to an amorphous phase while Co-rich compositions below Ni# ≤ 0.5 transformed to M-dittmarite NH4MPO4•H2O which coexisted with an amorphous phase (Figure ). Thus, the materials became phase-separated, despite originating from struvite solid solutions. Heated NixCo1-x-struvites with high Co content Ni# ≥ 0.8 showed no amorphous phases. In a hydrothermal synthesis in a saturated H2O environment at 90°C, Ni-struvite transformed to Ni-dittmarite NH4NiPO4•H2O (Figure ). Summing the thermal history of the compounds up, Mg-, Ni-and Ni-rich (Ni# > 0.5) NixCo1-x-struvites decomposed to amorphous phases in dry conditions, whereas Co-and Co-rich (Ni# ≤ 0.5 NixCo1-x-struvite transformed to the M-dittmarite component and other crystalline phases. The occurring phase transitions, crystalline-crystalline or crystalline-amorphous are strongly dependent on the humidity, absolute temperature, heating rate and on the choice of the metal cation in the struvite precursor. We show further that the evolution of amorphicity is a key property of the material. As a stable amorphous phase prevents the structural reconfiguration of the compound and the closing of the pores, it is a necessary condition for the development of mesoporous frameworks. Our results show that thermal evolution of M-struvites is driven by the removal of volatile components from the crystal structure (Figure , ESI: Figure ). This in turn, might make one to expect that porosity should develop in the materials over time. To test this, we characterized the evolution of porosity in our samples using gas adsorption. For the evaluation of BET surface areas and pore sizes, N2 gas adsorption-desorption measurements were performed on thermally-treated M-struvites (M = Mg, Ni, Co, and NixCo1-x). Based on these measurements, it was clear that the porosity would indeed develop at the mesoscale, but only in M-struvites which would become amorphous at the same time (i.e. Mg, Ni, NixCo1-x). On the other hand, Co-struvite and Co-rich NixCo1-x-struvite showing thermally induced crystalline transformations (Figure ) did not develop any significant porosity.