Abstract From the change in fracture mode as a function of distance from the specimen surface, the carbon diffusion distance can be estimated. In addition the diffusion parameter such as activation energy for carbon diffusion in molybdenum can be derived from Arrhenius plots of the carbon diffusion distance and the diffusion temperature. In this work the fracture surface of Mo–Ti alloys after carburizing was examined using a scanning electron microscope. Then the effects of Ti addition on the carbon diffusion behavior in molybdenum were discussed. It was demonstrated, first, the carbon diffusion distance decreases by adding Ti at a given diffusion temperature. Secondly the activation energy for carbon diffusion in Mo–Ti alloys is smaller than that in pure molybdenum. In addition the critical carbon concentration for the transition of fracture mode from intergranular to transgranular in Mo–Ti alloys is larger than that in pure molybdenum. These results are attributed mainly to the preferential combination between a carbon atom and a titanium atom.