We have characterized the output of inexpensive, commercially available single quantum well (SQW) blue and green light emitting diodes (LEDs). The SQW LEDs were amplitude modulated with the output from a frequency generator while biased through a bias tee with 5 mA of current. The blue SQW LED produced 800 &mgr;W of light centered at 466 nm, with a −3 dB bandwidth of 58 MHz. The green SQW LED produced 543 &mgr;W of light centered at 522 nm, with a −3 dB bandwidth of 26 MHz. Modulated light was available to approximately 100 MHz, allowing the measurement of ns fluorescence lifetimes. The fluorescence lifetime of a standard fluorophore (fluorescein) was measured in the frequency domain using the phase‐modulation technique, and gave results similar to those obtained with a 488 nm argon ion laser modulated with a Pockels cell. To demonstrate the usefulness of the SQW LED source, we also performed measurements with the fluorescentpH indicator SNAFL‐2. Again, these results compared favorably with those obtained with the laser. When compared to a laser modulated with a Pockels cell, the SQW LEDs were smaller, less expensive, required less power, generated less heat, and required less alignment. The ability to modulate the SQW LEDs at high frequencies, along with the blue or green outputs, allow their use as inexpensive light sources in fluorescence lifetime optical sensors and even fluorometers. ©1996 American Institute of Physics.