The structure of sputter-deposited Fe/Ti multilayers with various layer thickness and its change with annealing at elevated temperatures in vacuum and hydrogen atmosphere have been studied using X-ray diffraction and Auger-electron depth profiling techniques. The multilayers exhibit preferred orientations of Fe (110) and Ti (001) lying parallel to the substrates in the as-deposited states. When they are annealed in vacuum at 523-823 K, interlayer mixing takes place to form a supersaturated bcc solid solution and FeTi compound, and the original modulated structure almost disappears. No indication of solid-state amorphization is obtained during annealing. When the multilayers are annealed under 0.2-0.5 MPa H-2 in the same temperature range, the interlayer mixing is strongly suppressed and the original layer structure is almost preserved, although the Ti layers have transformed to those of delta-TIH2 or beta-TIHx depending on the temperature and H-2 pressure. This effect of hydrogenation on stabilizing the structure of Fe/Ti multilayers is well accounted for in terms of a calculated free-energy diagram For the Fe-Ti system.