The refractive index change caused by changes in the absorption edge of GaAs is determined by analysis of the spontaneous emission spectrum of a buried heterostructure window laser. The spontaneous emission spectrum is converted to a gain spectrum from which changes in the imaginary part of the refractive index can be calculated as the laser is excited from low current up to threshold. The real change in refractive index is then determined by a Kramers‐Kronig transformation. The change in refractive indexn′ of the GaAs active layer is slightly sublinear with minority carrier densitync. At the laser line, including the calculated contribution of free carriers, &Dgr;n′ = −0.025±0.005 anddn′/dnc= −(1.8±0.4)×10−20cm3in lasers with carrier densities at threshold estimated as 1.02×1018cm−3. Near threshold, the ratio of the change in the real index to the change in the imaginary index is about 6.2.