The yields of ionic and excited molecule reactions in γ-radiolysis of liquid propane from 35 to –130°C have been assessed by isotopic analysis of C3H8+ C3D8+ O2 and CH3CD2CH3+ O2mixtures. From a comparison with gas phase data the following conclusions are reached for the gas to liquid phase change : (a) the ionic decomposition yield decreases by 69%, (b) the net excited molecule decomposition yield decreases by 7% or less, (c) parent ion fragmentation still occurs in the liquid and exhibits fragmentation processes requiring from 1-4 eV excitation, (d) the H–2/H– transfer reaction ratio for C2H+4+ C3H8 seems to be increased, and (e) the isotopic decomposition of CH3CD2CH3 is drastically changed.