For the diatomic molecule AB in thermal and chemical equilibrium with its dissociated atoms, the pressure, density, specific entropy, and specific enthalpy are written as functions of the temperature and the fractional dissociation, with the approximations that the rotation is fully excited and there is no electronic excitation. These approximations are very good for many molecules over a large range of the variables. Atoms A and B can be either identical or distinct provided there is no excess of one atom over the other. For molecules composed of the isotopes of hydrogen, because of their large vibrational energies, it is possible to have considerable dissociation without vibration being important. If vibration is neglected, then the thermodynamic variables (written in dimensionless form) are related by equations which do not involve any properties of the species under consideration. Curves of constant pressure, density, specific entropy, and specific enthalpy are presented in the temperature‐dissociation plane. The errors introduced by neglecting vibration are proportional to exp (−TV/T), whereTVis the vibrational temperature.