SnS is a promising material for heterojunction solar cells, but the energy band alignment is not known for SnS-based heterojunctions. In this study, the energy band offset at the CdS/SnS heterojunction is calculated using the first principle, density-functional, pseudopotential method. A procedure analogous to that used in the core-level photoemission spectroscopy is adopted to calculate the band offset. The 4d core-level difference between Cd and Sn was estimated from the energy calculation of a superstructure consisting of zincblende CdS and rock-salt or zincblende SnS. The calculated valence-band offset is 0.1 eV when the rock-salt SnS is assumed and 0.84 eV when the zincblende SnS is assumed. (C) 2008 Elsevier B.V. All rights reserved.