The terminal solubility of hydrogen (TSH) in equilibrium with hydride in the Nb-Ta system is known to show a large intermediate maximum at temperatures in the region of 250 K. The origin of this “abnormal” behavior has not been successfully explained previously. In the temperature range of the TSH measurements, pseudo-binary (para-equilibrium) behaviour may be expected. Using the thermodynamic data available for the two pure metal-hydrogen systems, together with reasonable assumptions concerning the ternary mixtures, phase diagrams corresponding to complete equilibrium and paraequilibrium have been constructed for this system. It is clear from the phase diagrams that, rather than being anomalous, a large intermediate maximum in the TSH is the expected behaviour. It is suggested that an increase in the TSH on alloying is likely to be fairly general, irrespective of the relative affinity of solute and solvent for hydrogen.