The formation of Cd-Zn sulfide solid solutions from mixed hydroxides under hydrothermal conditions is investigated in detail. The work specifically aims to understand the formation and the role of nanotwinned mixed sulfide particles that have been reported to show excellent performance in photocatalytic water splitting (Liu, M.; et al. Energy Environ. Sci. 2011, 4, 1372). The influence of additives, pH, autoclave tumbling, and the state of the mixed hydroxide precursor on the mixed sulfides was studied by XRD, XPS, TEM, DR UV-vis, and N-2 physisorption. Cd-Zn sulfides are formed via a dissolution precipitation mechanism. Agitation of the synthetic medium and the formation of soluble intermediate complexes during hydrothermal treatment suppress the formation of a hexagonal wurtzite crystal phase and improve the photocatalytic activity of the mixed sulfides. The role of additives can be understood in terms of complex formation, pH maintenance, and adsorption on the facets of growing crystallites. All Cd-Zn sulfide samples exhibit compositional inhomogeneities, resulting in XRD line broadening and decreased bandgaps as compared with the values predicted by Vegard's law. Detailed TEM analysis revealed that the samples with higher amounts of nanotwinned particles were significantly less active in water reduction. The influence of nanotwinned particles is discussed in terms of extended crystal defects and charge carrier recombination.