Isothermal hydrogen desorption of spark discharge generated Mg/NbOx and Mg/Pd metal hydride nanocomposites is consistently described by a kinetic model based on multiple reaction rates, in contrast to the Johnson-Mehl-Avrami-Kolmogorov [M. Avrami, J. Phys. Chem. 9, 177 (1941); W. A. Johnson and R. F. Mehl, Trans. Am. Inst. Min., Metal. Eng. 135, 416 (1939); A. N. Kolmogorov, Izv. Akad. Nauk SSSR, Ser. Mat. 3, 355 (1937); F. Liu, F. Sommer, C. Bos, and E. J. Mittemeijer, Int. Mat. Rev. 52, 193 (2007)] model which is commonly applied to explain the kinetics of metal hydride transformations. The broad range of reaction rates arises from the disperse character of the particle size and the dendritic morphology of the samples. The model is expected to be generally applicable for metal hydrides which show a significant variation in particle sizes, in configuration and/or chemical composition of local surroundings of the reacting nanoparticles.