The theory of free‐carrier absorption is given for quasi‐two‐dimensional semiconducting structures when the carriers are scattered by nonpolar optical phonons and the radiation field is polarized in the plane of the layer. The absorption coefficient is calculated and is found to be an oscillatory function of the thickness of the layer and is enhanced over its bulk value for low values of thickness of the layer. Separate peaks associated with transitions involving phonon emission and absorption are observed. The results are interpreted in terms of phonon‐assisted transitions between size quantized subbands.