A novel route for the synthesis of complex metal sulfides using hydrothermal coupled dissolution-reprecipitation reactions is reported. Two thiospinels, (Ni,Fe) 3 S 4 (violarite) and Co 3 S 4 (linnaeite), were synthesized using (Fe,Ni) 9 S 8 (pentlandite) and Co 9 S 8 (cobaltpentlandite) as precursors. The Fe/Ni ratio of (Ni,Fe) 3 S 4 can be adjusted by varying the reaction conditions, for example, temperature (125-145 °C), pH (2.90, 3.90, 5.00), and precursor stoichiometry ((Fe x Ni 1-x ) 9 S 8 , x = 0.4, 0.5, 0.55, 0.6). Pure (Ni,Fe) 3 S 4 can be synthesized by utilizing a flow-through hydrothermal cell rather than a static hydrothermal cell as the fluid flow improves mass transfer and flushes away the Fe 2 O 3 byproduct from the reaction front. Synthesis times range from 10 to 20 days, compared to the traditional dry synthesis route for (Ni,Fe) 3 S 4 that requires 3 months annealing to obtain a product of only 72 ± 3 wt % purity. This synthesis route is ideal for preparing compounds with low thermal stabilities (<500°C).