DFT calculations have been carried out for Cu4Bi5S10 and Bi2S3 to provide an analysis of the relation between electronic structure, lone electron pairs and the local geometry. The effect of pressure is considered in Bi2S3 and the results are compared to published experimental data. Bi3+ in Cu4Bi5S10 is found at both symmetrically and asymmetrically coordinated sites, whereas the coordination environments of Bi in Bi2S3 are asymmetric at room conditions and get more regular with increasing pressure. The charge density maps of the asymmetric sites show the lone pairs as lobes of non-shared charge. These lobes are related to an effective Bi s–Bi p hybridization resulting from coupling to S p orbitals, supporting the modern view of the origin of the stereochemically active lone pair. No effective Bi s–p hybridization is seen for the symmetric site in Cu4Bi5S10, whereas Bi s–p hybridization coexists with a much reduced lone pair in Bi2S3 at high pressure.