The Bureau of Mines has sought clarification of the ti-tin system by studying the phases formed by diffusion reactions between titanium and titanium nitride. Reaction couples were prepared by embedding pockets of tin powder in arc-melted buttons of titanium. Specimens were vacuum annealed at nominal temperatures of 800 deg, 950 deg, 1,200 deg, and 1,500 deg c. Metallographically mounted cross sections from reacted specimens and from some that were not annealed were examined optically, with an electron microprobe, with an electron microscope, and by x-ray diffraction. Each annealed specimen contained a segregated core primarily of s-phase material, an e-phase diffusion layer, an a-phase diffusion layer, and host metal. On the basis of corresponding microprobe analyses, and considering results of certain other research, a new phase diagram is proposed. It contains no qualitative innovations, only quantitative adjustments. Results were also analyzed from the viewpoint of diffusion behavior. Diffusion coefficients calculated for the e-phase were sufficiently scattered that the parameters are very tentative. Nevertheless, the diffusion behavior reported is in reasonable agreement with one of two prior studies if it is assumed that the earlier investigators were mistaken in their identification of the nitride phase in which the diffusion occurred.