Abstract: Using a method of constant volume and alterable pressure, deuteration kinetics of TiHf_x(x=0.13, 0.26, 0.52) alloys and Hf were studied at the same initial pressures and in the temperature range of 473 to 873 K. The correlation of decrease of deuterium pressure with time was attained by the experiment of alloys absorbing deuterium. According to the results，the main factors that decide kinetics of Ti-Hf alloys absorbing deuterium were put forward and discussed. In the temperature range of 473 to 873 K, the deuteration curves of Ti-Hf alloys present two distinct figure in the higher or lower temperature region, which implies dissimilar reaction mechanism. Above 673 K, the α→β phase transition take place when Ti-Hf alloys react with deuterium gas. But that will be a α→δ phase transition as the temperature below 673 K. Thereby, it will be confirmed that the eutectoid phase transition temperature of TiHf_x-D system is about 673 K. With x of TiHf_x bigger, the eutectoid phase transition temperature become higher.