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A simple optical method for the quantitative determination of dissolved oxygen under physiological conditions is described. The technique involves measurement of room temperature phosphorescence from palladium(II) porphyrins. Such emission is rare for large organic molecules, and its long lifetime, together with the favourable absorption characteristics of the porphyrin receptor, ensure that the phosphorescence can be resolved easily, even in heterogeneous media. Phosphorescence yields and lifetimes are shown to be highly sensitive to the concentration of dissolved oxygen, but do not depend upon the nature of the environment, and can be used separately or together to determine oxygen levels within a biological substrate. This technique should be applicable to all areas of clinical, medicinal and biomedical chemistry.