The mobility of excess electrons in l-pentene, cyclohexene, 2-methyl-2-butene, and 2,3-dimethyl-2-butene has been measured as a function of pressure from 1 to 3000 bar and as a function of temperature. The volume changes associated with trapping of the electron in these alkenes are derived assuming an equilibrium exists between quasi-free and trapped electrons. The volume of trapping is positive or negative, depending on the magnitude of the electrostriction term relative to the cavity term. The entropy of trapping is small. A dependence of volume change on the compressibility of the liquid is shown.