Our results suggest that the reaction pathway is controlled by the type and concentration of the transition metal cation(s). Ni and Co can form the same isostructural crystalline phases, namely M-struvite and M-octahydrate. Therefore, looking at the final crystalline solids, one would also expect isostructural TMP solid solutions including both metals. However, the observed complex non-classical crystallization pathways of the considered TMPs suggest that the incorporation of both metals into crystals would be driven by very different kinetic rates and thermodynamic equilibria of different multi-step mechanisms. In this regard, the starting concentration of metals in solution is to reflect the metal concentration in the crystalline phases. Considering this hypothesis, in the further characterization, we explored if indeed any elemental fractionation occurred in the mixed NixCo1-x experiments between Ni and Co in their solid solutions. Therefore, to explain this aspect and to quantify the temporal elemental participation of P, Co, and Ni during the precipitation, we analyzed the chemical composition of the solid phases and the associated supernatants from different stages with ICP-MS. Although the measurements were essentially ex-situ, we used the same flow-through setup as with the scattering experiments to synthesize TMPs to ensure near-direct comparability among various approaches, evidenced also by matching the pH trends (ESI: Figure , Table ) The ICP-MS data of the mother supernatant correlate with the observations of the in-situ experiments (Figure , ESI: Table ). In the first seconds, Ni-and Co-PO4 co-precipitated near equally, as it is visible in comparable MP ratios of around 0.9 (Figure ). From scattering we learned that M-struvite formed quickly, while the amorphous phases began to agglomerate (Figure and Figure , ESI: Table ). Until around t = 3000 s the Ni/P remained constant, while the Co/P ratio showed a slight increase of around 10% overall (Figure ). In this period, Mstruvite and amorphous nanophases formed, as was indicated by the plateau value of the pH and the increasing sub-parameters in scattering (Figure , ESI: Figure -S3, Table ). After t = 3000 s the simultaneous dissolution of the primary M-struvite and the precipitation of Mphosphate octahydrate, resulted in a slightly increasing Ni/P ratio and starkly falling Co/P ratio. Based on the increasing G and Rg scattering parameters and the elemental analysis, the formation of the M-phosphate octahydrate crystals occurred gradually over two to three thousand seconds. At this stage, Ni was released slightly ≈ +15 % into the solution resulting in a Ni/P ratio of ≈ 1.05, while Co was almost completely ≈ -90% (decreasing Co/P ratio in the solution ≈ 0.05) incorporated in a solid phase (Figure ). Importantly, M-phosphate octahydrate had a higher MP = 1.5 than M-struvite with MP = 1, and therefore the absolute phosphate content increased after dissolution of M-struvite. Finally, after extended reaction times of t = 8000 s, the Ni/P ratio declined to a constant value of 0.9 reflecting only a partial incorporation of Ni into the M-phosphate octahydrate crystals. Most of the Co content was precipitated in M3(PO4)2•8H2O crystals, resulting in low remaining absolute Co content in the solution. Based on the EDS mappings, line scans, and bulk ICP-MS data from different NixCo1-x mixtures (x = 0.3, 0.5, and 0.7), we observed an extensive trend of metal fractionation during the formation of M-phosphate octahydrate (Figure ). The calculated Ni# from EDS analysis of the mapped M3(PO4)2•8H2O crystals were significantly lower than the Ni# in the solution (ESI: Figure ). Furthermore, in the EDS line scans, a slight zonation of Ni in the Co-rich samples could be identified (Ni# >0.5) in the M-phosphate octahydrate crystals (Figure ).