Thermodynamic properties of the solutions of hydrogen in metals and carbon in austenite were investigated and it is possible to predict the interstitial solubility in these systems over relatively wide ranges of temperature and solute activity. Monte Carlo methods were used to compute the arrangements of interstitial atoms on the tetrahedral sites in body centred cubic metals and the octahedral sites in face centred cubic metals. It was shown that the non-configurational entropies of solution of hydrogen in metals can be interpreted as being principally vibrational and on this basis the potential value of the isotopic solubility ratio theory in indicating the compositional variation of these non-configurational entropies was discussed. The need for future work along these lines is pointed out.