The energy transfer efficiency ratio, ETE=(σhΔEh/σxΔEx), for hydrocarbons dissolved in liquid xenon has the following values: ethane, 96; propane, 180; n-butane, 485; ethylene, 193; propylene, 255; cis-butene-2, 425; trans-butene-2, 665. The large increase on going from propane to butane is attributed to the energy absorption capability of the C2H5 groups through rotation about the central bond in butane. The presence of a double bond in the molecule enhances ETE, presumably through the participation of transient negative ion states in the energy transfer process. The cis–trans effect in butene-2 is still not understood.