Double sulfates of Co(II), Ni(II), Cu(II) and Zn(II) were obtained by evaporation of the reaction mixture of the corresponding metal sulfate and trimethylammonium sulfate aqueous solution in molar ratio 1:6, at room temperature. The crystals obtained were identified and investigated by elemental analysis, TG, DTG and DTA analysis, X-ray powder diffraction and electronic spectroscopy. It was found that the double sulfates of Co(II), Ni(II) and Zn(II) have the general empirical formula [(CH3)3NH]2M(SO4)2 . 8H2O, but although obtained in the same manner, the double sulfate of copper has the empirical formula [(CH3)3NH]2Cu2(SO4)3 . 7H2O. The X-ray powder diffraction patterns of the compounds obtained, show that the double sulfates of cobalt and zinc are isostructural, but although it has the same empirical formula, the double sulfate of nickel seems not to be isostructural with them. The thermal decomposition of the compounds is also discussed.