A model for the optical absorption ofa‐Si:H/a‐SixN1−x:H multilayers has been developed which takes into account the effects of alloying in the interface regions between thea‐Si:H anda‐SixN1−x:H layers. Four different spatial variations of the film composition at the interfaces have been considered: abrupt, step, linear, and parabolic. The optical response of the interface regions is proposed to arise from Si‐centered tetrahedra, containing both Si and N atoms. Calculations of the dielectric function of the multilayer using the appropriate effective medium approximation have indicated that alloying at the interfaces leads to a lower optical absorption and a broadening of the multilayer absorption edge as compared to multilayers in which interface alloying is ignored. However, interface alloying apparently does not contribute appreciably to the observed increase in the optical energy gap of thea‐Si:H layer in the limit of decreasing layer thickness.