A novel technique, which employs a modified Twyman–Green interferometer, is used to measure the nonlinear index of a CdS0.9Se0.1‐doped glass at different wavelengths in the vicinity of the band gap at room temperature. The measuredn2values for the doped glass are compared with calculated values obtained by Kramers–Kronig transformation of the measured &Dgr;&agr;(&lgr;)=&agr;(I1,&lgr;)−&agr;(I2,&lgr;). A good agreement is achieved between the measured nonlinear dispersion and that obtained through the Kramers–Kronig relations.