We describe a technique for direct measurement of the passive optical mode loss of a semiconductor laser or similar semiconductor waveguide structure, based upon measurement of the attenuation of optically excited luminescence in the guided mode as a function of distance traveled along the passive guide. A spectrometer is used to select luminescence in the low energy tail of the spectrum which is subject to very little reabsorption. We have applied the method to a series of highly strained GaInP quantum well laser structures and observe an increase in the mode loss from9.9 cm−1for 1&percent; strain to46 cm−1for 1.7&percent; strain. This correlates with the appearance of clustered regions in the highly strained wells observed by transmission electron microscopy (TEM). ©1997 American Institute of Physics.