Diffusion bonding of Ti-15 mol% Sn alloy to various iron materials was carried out in the temperature range of 1073-1273 K for 0.9-14.4 ks in a vacuum to verify interface separation, a phenomenon that has been observed between Ti-20 mol% Al alloy and high-carbon steel after bonding treatment at 1 273 K for 3.6 ks. Four types of carbon steel and one type of cast iron were used as an opposite material for the Ti-15 mol% Sn alloy. In the case of steel having a relatively low carbon content, a sound joint without defects like a gap at the interface was obtained through bonding treatment at 1 273 K for 3.6 ks. However, the joints with high-carbon steel and cast iron, which were fabricated at 1 273 K for 3.6 ks, had gaps at the interfaces, and several specimens separated near the interface promptly after bonding treatment. The separated surfaces of the high-carbon steel and cast iron were relatively smooth. A microstructure corresponding to the grain boundary and pearlite was observed in the surfaces. It was found that this phenomenon depended on the heating temperature and the holding time. These features were consistent with those of interface separation previously reported in the case of Ti-20 mol% Al alloy. Therefore, it is concluded that the separation phenomenon was caused by interdiffusion across the interface between the titanium alloy and the iron material.