The terminal solid solubility (TSS) of hydrogen in the Zr–2.5 wt. % Nb alloy has been determined by measuring the decomposition pressures of several Zr–2.5 wt.% Nb alloys containing hydrogen in the temperature range 425 to 900°C. The TSS boundaries α/(α+δ) and α/(α+β) were determined from the points of inflection of the van't Hoff decomposition pressure against temperature plots. The TSS of hydrogen in Zr–2.5 wt. % Nb alloy is about 1 to 2.5 atom % more than that in unalloyed zirconium (the maximum difference being at the eutectoid temperature of the Zr/H₂ system). The decomposition pressure against temperature relation in the α and (α+δ) phase regions and the temperature against concentration relation along the TSS boundary have also been derived. The mean relative partial molar enthalpy of hydrogen in the α-phase and the mean enthalpy of formation of δ-hydride from the saturated α-phase in the alloy are –24.6 kcal (mol H₂)^–1 and –40.9 kcal (mol H₂)^–1 respectively.