Reactions of HIr(CO)(P(p-tolyl)(3))(3) with SO2, O-2, mixtures of SO2 and O-2, and H2SO4 are described. Reactions with mixtures of SO2/O-2 and with H2SO4 lead to a common product, Ir2H4(mu-SO4)(P(p-tolyl)(3))(6), where a bidentate SO42- bridges the iridium centers. The complex HIr(CO)(P(p-tolyl)(3))(3) is stable toward O-2 but reacts readily with SO2 to give HIr(CO)(SO2)(P(p-tolyl)(3))(2), which was characterized by a crystal structure determination. The species Ir(CO)(H)(SO2)(P(p-tolyl)(3))(2) crystallizes in the centrosymmetric triclinic space group P (1) over bar with a = 10.675(1) Angstrom, b = 10.882(1) Angstrom, c = 17.740(2) Angstrom, alpha = 84.626(10)degrees, beta = 83.432(10)degrees, gamma = 87.043(10)degrees, V = 2036.6(3) Angstrom(3), and Z = 2. Diffraction data (Mo K alpha, 2 theta = 4.5-45.0 degrees) were collected on a Siemens R3m/V diffractometer, and the structure was refined to R = 3.00% for 4218 independent reflections with F-0 > 6 sigma(F-0). The five-coordinate iridium(I) center has a distorted environment (closer to trigonal bipyramidal than to square pyramidal) with H(1)Ir(1)-S(1) = 171.4(23)degrees. Bond lengths are Ir-P = 2.303(2) and 2.320(2) Angstrom, Ir-SO2 = 2.372(2) Angstrom, Ir-CO = 1.906(8) Angstrom, and Ir-H = 1.65(7) Angstrom. The SO2 ligand has bond distances of S(1)-O(1) = 1.442(7) and S(1)-O(2) = 1.441(7) Angstrom, with O(1)-S(1)-O(2) = 113.1(4)degrees.