The phase transitions in mixed crystals of the system K1-x(NH4)(x)SCN for a wide range of x values have been studied using differential scanning calorimetry (DSC). On the basis of the experimental data a phase diagram was constructed. The diagram contains two curves : (a) - the line of the order-disorder phase transition separating regions of the orthorhombic (phase II) phases, (b) - the line of coexistence of the monoclinic (phase III) and the orthorhombic one. The enthalpies of both transitions were determined. Careful investigations of the thermal hysteresis of the phase II-phase I transition showed no systematic dependence on the concentration. For most concentrations Delta(hyst) T approximate to 0 and does not excite 0.8 K for any concentration. For pure ammonium thiocyanate the kinetics of the spontaneous transition of undercooled phase II into stable phase III has been investigated.