The single-phase KZn2(PO4)(HPO4) was synthesized via solid-state reaction at 80 degrees C using K3PO4 center dot 3H(2)O, K2HPO4 center dot 3H(2)O, and ZnSO4 center dot 7H(2)O as raw materials. The thermal decomposition of KZn2(PO4)(HPO4) experienced one step, which was the intramolecular dehydration of the protonated phosphate groups to form Zn2P2O7. The apparent activation energy E-a was estimated with six comparative isoconversional procedures. The average value of the apparent activation energy E-a associated with the thermal decomposition of KZn2(PO4)(HPO4) was determined to be 411.49 +/- 14.37 kJ mol(-1), which demonstrates that the thermal decomposition of KZn2(PO4)(HPO4) is a single-step kinetic process and can be described by a unique kinetic triplet [E-a, A, g(alpha)]. A new modified method for multiple rate isotemperature was used to define the most probable reaction mechanism g(alpha), and reliability of the used method for the determination of the kinetic mechanism was tested by the comparison between experimental plots and modeled results for every heating rate. The results show that the mechanism function is reliable. The value of pre-exponential factor A was obtained on the basis of E-a and g(alpha). Some thermodynamic functions (Delta S double dagger, Delta H double dagger, and Delta G double dagger) of the transition state complex were also calculated.