Abstract—Kinetics of inactivation of two separate leucine transport systems (leucine specific and LIV) in E. coli by seven wavelengths of monochromatic light have been studied. Loss of leucine uptake is not due to generalized membrane damage causing non‐specific leucine leakage. Inactivations are usually exponential but some wavelengths show shoulders at low doses. Two‐component spectra between 254 and 435 nm occur for both transport systems. Inactivation is most efficient at 290 nm and a second peak occurs at 365 nm. Both leucine transport systems are inactivated similarly.