The photochemistry of a series of alkene-O-2 complexes was studied in a supersonic expansion using a resonance enhanced multiphoton ionization probe of the O(P-3(j)) photoproduct at 226 nm. The relative yield of oxygen atoms from each complex was correlated to the ionization potential of the alkene species and indicates that initial excitation of an intermolecular charge-transfer state mediates the subsequent excited state chemistry. The behavior is similar to that observed previously for the C6H6-I-2 system: a reverse electron-transfer step yields electronically excited O-2 which subsequently dissociates. The kinetic energy release of the O(P-3(j)) fragment was also measured using a time-of-flight analysis and found to be small with an isotropic spatial distribution. No evidence for photo-oxidation of the alkenes was observed in the mass spectra. A comparison is made to the charge-transfer absorption spectra observed in cryogenic oxygen matrices of similar alkene complexes. Ab initio models were used to identify the stable ground state geometry of the C2H4-O-2 complex and complete active space self-consistent-field calculations were performed to identify the energy of the charge-transfer state for several alkene-O-2 complexes.