Single molecules are detected through the phosphorescence emission of their triplet states. Emission of the triplet states of single molecules of Pt octabutoxycarbonyl porphyrin (PtOBCP) and ruthenium(II)-tris-4,7-diphenyl-1,10-phenanthroline dichloride (Ru(dpp)(3)Cl-2) is reported. The single molecule phosphorescence is very sensitive to molecular oxygen. Each molecule has its own characteristic quenching rate by oxygen, and the distribution of these rates is measured for (Ru(dPP)(3)Cl-2) on a quartz surface. The large variance of this distribution is presumed to be caused by fluctuations in the pseuclobimolecular rate coefficient and the local oxygen concentration. The possibility of creating a quantitative single oxygen molecule sensor is suggested.