A technique is described for measuring the photon absorption coefficients in the active and passive regions of an electron‐beam‐pumped semiconductor laser. Measurements made at 4°K onn‐type, single‐crystal GaAs indicate that photon loss in the active region is due to free carrier absorption, and that this absorption coefficient increases linearly from 2.5 cm−1to 47 cm−1as donor concentration is varied from 2.4 × 1017cm−3to 4.7 × 1018cm−3. For each case the active region absorption coefficient is less than that measured in the passive region, which varies from 354 cm−1to 98 cm−1over the same range of donor concentrations. The increased loss in the passive region is attributed to interband absorption transitions.