We study the Burstein-Moss shift and related properties for degenerate n-type semiconductors forming band-tails, on the basis of a newly formulated electron dispersion law and taking into account the wave vector dependence of the optical matrix element. It is observed, taking n-InSb as an example for numerical computations, that the optical activation energy increases with increasing carrier concentration and the present theory exhibits better agreement with experimental findings than that of the conventional approach based on the wave vector independence of the optical matrix element. © 2003 Elsevier B.V. All rights reserved.