The desorption kinetics of CaNi5, LaNi5 and LaNi4.7Al0.3 hydride were determined and compared under isothermal conditions and at equivalent constant-pressure thermodynamic driving forces. At 40 °C the desorption plateau pressure of LaNi5 was 3.8 atm whereas the other two materials both had plateau pressures of 0.9 atm. It was initially believed that, since LaNi5 had a higher plateau pressure and formed a less stable hydride than the other two alloys, it would release its hydrogen the most rapidly and the other two materials would release hydrogen at about the same rate. Under the conditions used, the reaction rates varied in the order LaNi5 LaNi4.7Al0.3 CaNi5. The fact that LaNi4.7Al0.3 reacted more rapidly than CaNi5 could not be explained solely on the basis of hydride stability. It was believed that other factors, such as the differences in the particle size distributions among the various hydrides, or differences in the nature of their surfaces, may also have an effect on the kinetics. An examination of the particle size distribution in each material by laser light scattering spectroscopy showed that LaNi4.7Al0.3 had a much smaller particle size distribution than did CaNi5. Auger electron spectroscopy revealed that the percentage of nickel on the surface of CaNi5 was lower than those on the other two alloys. These results indicate that a small particle size distribution and the presence of nickel clusters on the surface are factors which enhance the rate of hydrogen desorption.