Abstract—The production of singlet oxygen was measured indirectly for three classes of photosensitizers: porphyrins (Photofrin II, TPPS4), chlorins (MACE, DACE), and a phthalocyanine (CASPc). Buffered solutions of sensitizers and singlet oxygen acceptors were irradiated with a CW dye laser and the oxygen depletion was monitored electrochemically with a Clark‐type microelectrode. A comparison of oxygen‐depletion rate constants and quantum efficiencies yields the order of efficiency of the sensitizers: TPPS4>MACE>PII>DACE>CASPc. For singlet oxygen acceptors the order was: furfuryl alcohol>imidazole>tryptophan. CHO cell suspensions were also used as acceptors. Here the order of efficiency (per absorbed photon) was PII>MACE = CASPc. Expressed in terms of oxygen depletion per cell the order was CASPc = PII>MACE. When performing cell clonogenicity studies the order of efficiencies, expressed as percentage cell kill per unit weight of sensitizer, was CASPc>PII>MACE = DACE. The discrepancy between the efficiencies of sensitizers to generate singlet oxygen and their cytotoxicity was explained in terms of photodegradation (for the chlorins), intracellular localization (for PII), and contributions from a Type I mechanism (for C