Ab initio calculations have been used to study the effects of hydrogen on vacancy concentrations in α-Fe and Ni. The presence of H interstitials aided vacancy formation in both metals but via two different mechanisms. In α-Fe, trapping of H by a vacancy is favourable. However binding of further hydrogen atoms was not predicted to proceed. The thermal equilibrium concentration of H interstitials in comparison to vacancies in α-Fe is many orders of magnitude higher over a wide temperature range. Excessive H interstitials in solid solution facilitate vacancy formation, lowering the required energy by 0.79eV (down to 1.41eV). In Ni, a single H interstitial is not expected to have an impact on the vacancy population, increasing the vacancy formation energy by 0.32eV. Two bound H interstitials however are predicted to decrease the vacancy formation energy by 0.52eV with expected arrangement along a 〈111〉 direction around the vacancy. The calculations show reasonable agreement with experimental data when comparing crystal lattice contractions of the pure metals and predicted melting temperature of the Me–H alloys.