The hydriging reaction kinetics of LaNi4.7Al0.3 under isothermal conditions were studied in detail using a double-walled reactor which possessed high heat conductivity. The reaction characteristics in the alpha+beta phase region were discussed from the viewpoint of rate-controlling steps. It was found that several rate-controlling steps were involved during hydriding reactions from the alpha to the beta phase regions and the rate-controlling step changed with the change of phase regions. For the hydriding reaction in the alpha+beta phase region, the rate-controlling step was found to be the phase transformation process at the interface between the alpha and the beta phases. The hydriding reaction kinetics of other rare earth-nickel based alloys, LaNi5-xAlx (x = 0, 0.1, 0.2) and MmNi(4.5)Al(0.5) (Mm:mischmetal), were determined in the same way, and similar results were obtained.