Phosphorescence lifetime of phosphors dissolved in biological fluids is dependent on the local oxygen concentration. When phosphor is added to the blood in vivo there is a distribution of lifetimes due to the distribution of oxygen concentrations in the blood of the sampled tissue volume. The distribution of oxygen can be assessed non-invasively and in practically real-time using frequency domain time-resolved phosphorometry. Here we describe a multi-frequency system for measurements of oxygen distributions in tissue in vivo.