The linewidth enhancement factor &agr; has been determined in GaInSbAs/GaSb lasers emitting near 1.8 &mgr;m (2 &mgr;m at RT) from the spontaneous emission spectra below threshold. For type‐I lasers, where ap‐njunction was located at the GaAlSbAs/GaSb heteroboundary, the measured &agr; is 3.1 at the lasing wavelength at 82 K. In type‐II lasers with thep‐GaSb/GaInSbAs heterojunction &agr; was estimated to be at least 2 times lower, that can be attributed to the formation of a quantum well structure at this heteroboundary under lasing conditions.