We present a new detection instrument for sensor measurements based on excited-state fluorescence lifetimes. This system consists of a primary optoelectronic loop containing a resonance-type rf amplifier, a modulatable fluorescence-excitation light source, a fiber optic feedback loop (with a gap for a fluorescent sensor), and a photomultiplier tube. A secondary, phase-feedback optoelectronic circuit consists of a long-wavelength-pass optical filter, a second photomultiplier tube, a photodiode, an electronic phase detector, a dc amplifier, and an electronic phase shifter (inserted into the main loop). This phase-feedback circuit is new with respect to our previous work. Under the appropriate conditions, the main loop exhibits self-oscillations, manifesting themselves as sinusoidal rf modulation of light intensity. The phase-feedback circuit detects the modulation phase shift resulting from the finite excited-state lifetimes of a fluorophore. As the excited state lifetime changes, the phase shift from the electronic phase shifter also changes, which results in a shift in self-oscillation frequency. The detection system uses self-oscillation frequency as the detection parameter and has excellent resolution with respect to changes in excited-state lifetime (∼1 ps). © 2004 Society of Photo-Optical Instrumentation Engineers.