We describe measurements of the antiguiding parameter, α, for several optically pumped semiconductor lasers. Three laser structures were investigated; two of the lasers utilize W-quantum wells (QWs) in which 14 InAs/In0.4 GaSb/InAs QWs are imbedded in lattice-matched In0.25 GaAsSb layers. The emission wavelengths of the W lasers were ∼3.5 and 4.5 μm, respectively. The other laser, a double heterostructure (DH) design, contained a ∼1.5 μm InAsSb active region embedded in ∼2.5 μm thick AlAsSb clad regions. The emission wavelength of the DH was λ∼3.8 μm. We employed the Hakki–Paoli method [B. W. Hakki and T. L. Paoli, J. Appl. Phys., 44, 4113, (1973)] in conjunction with a Fourier transform infrared spectrometer to measure subthreshold gain and index variations as a function of pump intensity. To reduce errors associated with incoherent background emission a full spectral curve fit was used to determine the differential gain and index. The results reveal the antiguiding factor in the W lasers to be low with α∼1.0. The antiguiding factor for the DH was markedly larger with α=9.4±1.3. We attribute the low α for the W lasers to the higher QW gain as well as to inhomogeneous broadening induced by the 14 QWs. The differing well widths and the independent optical pumping of the wells, leads to a net gain spectrum that is symmetrical about the gain peak. This symmetry, in turn, leads to small differential index shifts at the gain peak; the result of the small differential index and large differential gain is low antiguiding