The energy of activation for viscous flow at constant volume, Ev, is dependent on the volume of a liquid. For normal liquids at elevated temperatures, Ev is only a small fraction of the value of the activation energy at constant pressure, Ep. However, for associated liquids, on account of the presence of hydrogen bonds, Ev is almost equivalent to Ep. It is suggested that in highly associated liquids like liquid silica and some molten glasses where flow necessitates the rupture of bonds of even greater energy, the difference between Ev and Ep should be negligible.