Solutions of porphyrin (H2L) complexes, RuL(MeCN)2, in MeCONMe2, DMF, or pyrrole, bind 1.0 mol of O2 per Ru reversibly at 20 oC and 1 atm pressure. The MeCN complexes are prepared by photolysis of the corresponding RuL(CO)(EtOH), complexes in MeCN. In PhMe solution, the O2 reaction leads only to a slow irreversible oxidation to Ru(III). Related complexes with more strongly coordinated axial ligands, such as pyridine and N-methylimidazole, also give only oxidation products. Electrolysis of the bisacetonitrile complex brings about a reversible 1-electron oxidation to give a Ru(III) cation.