Time-resolved singlet oxygen, O(2)(a(1)Delta(g)), phosphorescence experiments have been performed in single cells upon pulsed laser irradiation of a photosensitizer incorporated into the cell. Data recorded as a function of the partial pressure of ambient oxygen to which the cell is exposed reflect apparent values for the intracellular oxygen diffusion coefficient and intracellular oxygen concentration that are smaller than those found in neat H(2)O. This conclusion is supported by O(2)(a(1)Delta(g)) phosphorescence data and sensitizer triplet state absorption data recorded in control experiments on sucrose solutions with different viscosities. We recently demonstrated that the intracellular lifetime of O(2)(a(1)Delta(g)) is comparatively long ( approximately 3 micros) and does not differ significantly from that in neat H(2)O ( approximately 3.5 micros). Despite this long lifetime, however, our estimate of an apparent intracellular oxygen diffusion coefficient in the range approximately 2-4 x 10(-6) cm(2) s(-1) means that the spatial domain of intracellular O(2)(a(1)Delta(g)) activity will likely have a spherical radius of approximately 100 nm. This latter point helps reconcile seeming inconsistencies between our direct O(2)(a(1)Delta(g)) lifetime data and results obtained from independent photobleaching experiments that show a limited translational diffusion distance for O(2)(a(1)Delta(g)) within a cell.