Quantum yields and efficiencies of O-1(2) ((1)Delta(g)) production along with photophysical properties for a number of Re(I) complexes in acetonitrile solutions are reported. Two different classes of Re(I) complexes, L-S-CO2-Re(CO)(3)(bpy) (L-S = 2-pyrazine, 2-naphthalene, 9-anthracene, 1-pyrene, 2-anthraquinone) and XRe(CO)(3)L (X = CF3SO3, py; L = bpy, phen), were probed as photosensitizers for O-1(2) ((1)Delta(g)) production in air-saturated acetonitrile solutions. Depending on the nature of the Re(I) complex, the excited state responsible for the generation of O-1(2) ((1)Delta(g)) is either a metal-to-ligand charge transfer ((MLCT)-M-3) or a ligand centered ((LC)-L-3) state. With L-S-CO2-Re(CO)(3)(bpy) complexes, O-1(2) ((1)Delta(g)) is produced by oxygen quenching of (LC)-L-3 states of anthracene and pyrene with high quantum yields (Phi(Delta) between 0.8 and 1.0), while the complexes bearing the ligands L-S = 2-anthraquinone, 2-pyrazine, and 2-naphthalene did not yield O-1(2). XRe(CO)(3)L complexes generate O-1(2) ((1)Delta(g)) mainly by oxygen quenching of their (MLCT)-M-3 luminescence with an efficiency of O-1(2) ((1)Delta(g)) formation close to unity. Bimolecular rate constants for the quenching of the XRe(CO)(3)L complexes' emission by molecular oxygen range between 1 x 10(9) and 2 x 10(9) M-1 s(-1), and they are all <= 1/9k(d), in good agreement with the predominance of the singlet channel in the mechanism of O-1(2) ((1)Delta(g)) generation using these Re(I) complexes as photosensitizers. All the experimental singlet oxygen efficiencies are consistent with calorimetric and luminescence data for the studied complexes. With L-S-CO2-Re(CO)(3)(bpy) complexes, calorimetric experiments were utilized in the calculation of the quantum yields of triplet formation; namely phi(T) = 0.76 and 0.83 for the triplet states of anthracene and pyrene, respectively.