Abstract Previously developed relationships between isomobility states and equilibrium p- v- T properties of vinyl polymers are extended to predict mobilities, μ, at constant temperature and at constant volume, with poly(vinyl acetate) as an example. At constant volume, μ changes by several orders of magnitude while the ‘internal pressure’ remains constant, suggesting that kinetic energy (temperature) dominates in governing μ. From μ at constant temperature the Vogel parameters, B and T 0, are found to increase with pressure, the former increasing linearly. A new Vogel type equation is developed in which one of the parameters, B v , depends only on the chemical composition of the polymer. Both μ and its ‘activation energy’ at constant pressure, E p , are shown to be constant at the glass transition.