Abstract The cation substituted Tutton's salt single crystals were obtained with empirical formula K 2 Ni x Co 1-x (SO 4 ) 2 •6H 2 O and K 2 Ni x Mg 1-x (SO 4 ) 2 •6H 2 O (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0). All crystals were grown by slow evaporation technique and investigated by X-ray powder diffraction, differential thermal analysis, UV–VIS–NIR, and atomic absorption spectroscopy. Combination of differential thermal analysis (conducted in 25–840 °C temperature range), X-ray powder diffraction and mass loss measurement adds information about thermal stability and phase composition at different temperatures. The optical characteristics such as transmittance and optical energy gap of the grown crystals were determined. Established, that K 2 Ni 0.4 Co 0.6 (SO 4 ) 2 •6H 2 O crystals has a highest dehydration temperature (137 °C), band gap energy (6.49 eV) and transmittance in UVB region. • Single crystals of K 2 Ni(SO 4 ) 2 .•6H 2 O, K 2 Co(SO 4 ) 2 •6H 2 O K 2 Mg(SO 4 ) 2 × 6H 2 O and K 2 Ni x Co 1-x (SO 4 ) 2 × 6H 2 O, K 2 Ni x Mg 1-x (SO 4 ) 2 × 6H 2 O solid solutions were grown. • Dehydration and decomposition processes were investigated by differential thermal analysis, XRD and mass loss measurement. • Optical characteristics of single crystals were determined.