The physical origin of the equation of state (EOS) used by Vinet et al. has been discussed in view of the fact that this EOS is based on the Rydberg interatomic potential function as recently found by Stacey. The Rydberg EOS has been generalised so as to make it consistent with the infinite pressure extrapolation such that the pressure derivative of isothermal bulk modulus K′ T and the ratio K T /P both tend to K′ ∞ , the value of K′ T in the limit of infinitely large pressure. The validity of the generalised Rydberg EOS has been tested using the results based on the Hama and Suito model and the Stacey reciprocal K-primed EOS.