The applicability of an unconventional approach in the formal kinetic analysis of solid-state reactions is discussed. This approach is based on the joint use of so-called isoconversional methods and of the non-linear regression analysis. By means of this approach reliable kinetic parameters for the phase transitions between three polymorphic modifications of calcium carbonate were obtained. The influence of the conditions of sample preparation on the transition kinetics has been studied. Two different transition mechanisms are compared using the calculated kinetic data. The isoconversional method, proposed by Friedman, delivers activation energy values, which deviates from data, obtained by means of the Flynn-Wall-Ozawa method. Additionally, the calculated apparent activation energy is dependent on the degree of conversion. The influence of some model deviations on the accuracy of the isoconversional methods is discussed. It is shown that the robustness of these methods towards model deviations may be tested by an analysis of theoretical plots.