The question arose how a mixed-metal system evolves towards crystalline solid solutions. We initially hypothesized that the development rate would be within the limits established by pure metal systems. As a result, compositions containing more Ni would develop faster than those with more Co. In contrast, the time for completion of the reaction increased strongly and differed significantly from the behavior of the two individual endmembers. We considered several Ni:Co ratios (Ni# = 20, 50, 80) at several concentrations (c(M) = 0.02 M, 0.1 M, 1 M). In this regard, the most counterintuitive mixture was the 50:50 one that exhibited the longest equilibration times (ESI: Figure ). This anomalous non-linear (related to the Ni#) trend is further pronounced at very high concentrations of 1 M of the reactants, where the Ni0.5Co0.5system required 6 h to complete, while the pure endmembers needed at most less than one quarter of this time (ESI: Figure and).