The single-crystals of mixed (C3H5N2)2[KFexCo1-x(CN)6] crystals, with different ratios of x = 0, 0.29, 0.42, 0.51, 0.63, 0.70, 0.85, 1, have been grown from aqueous solutions. The thermal stability of the crystals has been determined using both DTA and TGA analysis. DSC has revealed sequences of structural phase transitions (PTs). We have found four solid phases for the crystals with concentrations x < 0.6 and three phases above these concentrations. The Fe(iii) concentration has been estimated using the SEM technique. We have found the linear relationship of Tcversus the molar concentration of Fe(iii) for the IV → III PT. Based on the obtained results, a phase diagram has been constructed. The mechanism of the structural PTs has been discussed based on the results of dielectric relaxation, NIR, IR and Raman spectroscopyand X-ray measurements. Dielectric switching has been observed for all analysed mixed-crystals. All crystals exhibit ferroelastic properties.