Valence electron and positron charge densities in YN and ScN are obtained from wave functions derived in a model pseudopotential band structure calculation coupled with the independent particle model (IPM). it is observed that the positron density is maximum in the open interstices and is excluded not only, from the ion cores but also to a considerable degree from the valence bonds. Electron-positron momentum densities are calculated for the (001) and (110) planes. The results are used to analyze the positron effect in binary alloy semiconductors. (c) 2008 Elsevier B.V. All rights reserved