A new equation of state for hydrogen gas (0 <= P <= 10(11) Pa, 298 <= T <= 1000 K) has been derived and is given in an analytical form It is compatible with the Calphad approach (i e it is an analytical equation of the Gibbs energy as a function of pressure) This equation has been applied to the Calphad assessment of the H-Rh system, an example of system forming a metal hydride only at a very high pressure A set of self-consistent thermodynamic parameters describing the Gibbs energy of each phase has been obtained. The scarce experimental data available for this system, particularly the high formation pressure of the hydride, have been well reproduced which otherwise would have been impossible without considering the non-ideal behaviour of the gas phase (C) 2010 Professor T Nejat Veziroglu Published by Elsevier Ltd All rights reserved.