Fiber optic oxygen sensors based on polynuclear aromatic hydrocarbons or transition-metal complexes require excitation light in the UV and blue region of the spectra (1-14,16,18,19). These types of oxygen sensors are generally intensity modulated and are therefore sensitive to perturbations such as bending of the optical fiber and photodegradation of the dye. Oxygen sensors based on emission decay time measurements (15,17,20,21), which would overcome these problems, have been developed; but these sensors require complicated electronics. The oxygen sensor described here is based on a ratiometric emission intensity measurement of two porphyrins, which minimizes the effect of environmental perturbations on the optical signals (23). The excitation and emission spectra of the dyes are in the green, orange, and red region of the spectra, which makes the sensor compatible with LED excitation and photodiode detection.