Abstract The solubility and reactivity of seven transition‐metal sulfates (Cr(III), Mn(II), Fe(II), Fe(III), Co(II), Ni(II), Cu(II)) in the ternary eutectic (78 mol‐% Li2SO4, 8.5 Na2SO4, 13.5 K2SO4) and in the ternary eutectic containing NazCOa have been studied and the transition‐metal oxide products identified.