In the low density vapors the density normalized mobilities μnof thermal electrons decreased in the ordern-hexane > cyclopentane > cyclohexane, although the differences were only ∼ 10%. The mobilities began to increase at electric field strengthsE/n > 0.4 Tdin cyclopentane, > 0.5 Tdin cyclohexane and remained independent of field strength up to 1.5 Tdinn-hexane. The ratio of the threshold drift velocity for electron heating to the speed of low frequency sound in the gas, υd(threshold)/c0 = 11 in cyclopentane, 14 in cyclohexane, and is > 35 inn-hexane; it increases with decreasing sphericity of the molecules. The electrons are cooled mainly by inelastic collisions with the hydrocarbon molecules. The temperature coefficients of mobilities in the low density gases increase as the molecules become more globular, which could reflect the participation of either low lying transient negative ion states or a Ramsauer–Townsend effect in the scattering processes. The normalized mobilities μnin the saturated vapors began to decrease atn = 4 × 1019 molecules/cm3inn-hexane, 13 × 1019in cyclohexane, and 30 × 1019in cyclopentane. It appears that the minimum size of molecular cluster required for electron quasilocalization is smaller inn-hexane than in cyclohexane, and larger in cyclopentane. Electron localization interactions are weaker for more globular molecules.Mobilities in the critical fluids were 16 cm2/V s inn-hexane, 23 in cyclohexane, and 22 in cyclopentane.Mobilities in the liquids were independent of field up to the highest value used, which was 1.5 Tdin the hexanes and 0.9 Tdin cyclopentane. The mobilities and their temperature dependences were interpreted in terms of a model