Solid solutions of general formula K x(NH 4) 2- xMn 2(SO 4) 3 (0 < x < 2) and K yTl 2- yMn 2(SO 4) 3 (0 < y < 2) have been prepared. All of the phases within the composition range studied have been established, and their cubic symmetry at room temperature has been confirmed. The cell parameters for each member of the solid solutions have been determined. The substitution has been found to be homogeneous. Differential scanning calorimetry experiments of solid solution with x = 2.00, 1.81, and 1.49, and y = 2.00 and 1.79 have been carried out in order to study the transition mechanism in the ferroelastic langbeinite K 2Mn 2(SO 4) 3. The Tc for the phase transition P2 13- P2 12 12 1 is -75.9°C. The mixed crystals of NH +4 show phase transition up to 10% substitution, with a decrease in both Tc and ∆H. On the other hand, the phase transition disappears in the mixed crystals of Tl +. The size of the M+ ion plays an important role in the phase transition.