AbstractThe thermodynamic properties of the hydrogen solution in amorphous Fe ‐ Ti films (Ti: 43–60%) have been studied in the pressure range from 1 to 104 Pa and at temperatures from 293 to 623 K. It is shown that the observed pressure‐composition (PC) isotherms deviate positively from the Sievert's law, which is contrary to the negative deviation commonly observed in the hydrogen solution in crystalline compounds. In accordance with the positive deviation the relative partial molar enthalpies, ‐ Δ H̄, derived from the isotherms decrease with increasing the hydrogen concentration, and at very dilute concentration the value is about three times larger than that for the formation of monohydride of crystalline FeTi. These anomalously large ‐ Δ H̄ values for hydrogen dissolution in amorphous Fe‐Ti suggest that the local environment around the hydrogen atoms are quite different from that in crystalline FeTi. The relative partial molar entropy data are analyzed taking into account of the contribution from the vibrational and fromielectronic heat capacity and volume expansion terms. The results show that the number of interstitial sites available for hydrogen occupation at very dilute concentration is about five times smaller than the values observed in the V‐, Nb‐, Ta‐H systems.