A quantity derived from the pressure dependence of the rate constant of a reaction (mainly used for reactions in solution), defined by the equation: $${\mathrm{\Delta }}^{‡}V=-RT(\frac{\partial (\ln k)}{\partial p}{)}_T$$ providing that the rate constants of all reactions (except first-order reactions) are expressed in pressure-independent concentration units, such as $\mathrm{mol}{\mathrm{dm}}^{-3}$ at a fixed temperature and pressure. The volume of activation is interpreted, according to transition state theory, as the difference between the partial molar volumes of the transition state ($V$) and the sums of the partial volumes of the reactants at the same temperature and pressure, i.e. $${\mathrm{\Delta }}^{‡}V{=}^{‡}V-\sum \left(r{V}_{\text{R}}\right)$$ where $r$ is the order in the reactant R and $V_{\text{R}}$ its partial molar volume.