Chlorophyll a (Chl a) as a lipophilic photosensitizer can induce biomembrane destruction via Type-II reaction involving singlet oxygen (¹O₂) as a primary initiator. Type-I oxidation by an excited-state photosensitizer reacting directly with lipid substrate also contributes but the primary intermediate remains to be verified experimentally. We have investigated the reaction dynamics initiated by Chl a-photosensitization involving oxygen and the -C=C- moieties of lipids in the membranes of small unilamellar vesicles (SUVs) prepared from phospholipids of different degree of unsaturation (0, 1 or 2). Under anaerobic condition, femtosecond time resolved absorption (fs-TA) combined with spectroelectrochemical spectroscopies validated the formation of Chl a^[rad]− in subpicoseconds, and time resolved fluorescence spectroscopy revealed the rapid quenching of ¹Chl a* with a rate constant of (28 ns)⁻¹. These ultrafast processes, independent of oxygen, are ascribed to the electron transfer reaction from a -C=C- moiety to ¹Chl a* as an initiation step of Type-I reaction. On longer timescales, ns-TA spectroscopy unraveled the drastic quenching of ³Chl a* by either -C=C- or O₂, and the quenching by O₂ was found to be 20 times more efficient. This together with the ¹O₂-luminesence analysis prove the involvement of both types of photosensitization. In addition, HPLC-MS spectroscopy confirmed the ketonic, the alcoholic and the core-aldehyde products of lipid oxidation. Moreover, the oxygen dependent partition between Type-I and Type-II reactions is discussed on a detailed kinetics basis, showing that the two mechanisms of lipid oxidation are equally important under an oxygen concentration of 1.3×10⁻⁵ M at room temperature.