In this work, we investigate the effects of the insertion of a light element in the vacant sites of BP. Calculations have been performed by means of the ab initio pseudo-potential method with the local density approximation (LDA), using additionally the interstitial sites. A predictive study is presented for the charge density distribution at various k-points and for various conduction bands. We show that the X-C point moves up in energy relative to the Gamma(C) point when closed-shell atoms (H in this case) are substituted at the interstitial sites. Results indicate a change of the optical and mechanical properties for the resulting compound. (C) 2002 Elsevier Science B.V. All rights reserved.