The structural and electronic properties of polysilane, (SiH2)x and polysilene, (SiH)x will be compared with their carbon-based analogues polyethylene, (CH2)x, and polyacetylene, (CH)x, as investigated by the ab initio crystal orbital method using several, partly highly polarized, atomic basis sets of increasing size at the Hartree–Fock (HF) level and by including electron correlation effects up to the third order of Møller–Plesset perturbation theory. The single-particle energy bands have also been corrected for correlation effects applying the electron polaron quasiparticle (QP) method. Electron correlation plays an essential role in stabilizing the gauche conformation of polysilane against the trans form. While the gauche–trans energy difference is about 0.25 mH at the HF level, it will be reduced to about -0.8 mH due to correlation. Also the energy bands and band gaps will be substantially modified in going from the HF to the QP description, resulting in ΔEgap = 9.5 and 11.8 eV for trans and gauche polysilane at HF, and 5.12 and 5.56 eV at the QP level, respectively. Polysilene turns out to be a semiconductor (similar to polyacetylene) for which bond-alternation energetically stabilizes the semiconducting phase (as compared to the equidistant, metallic one) by 1.4 mH at the HF and by 0.6 mH at the MP2 levels, with bond alternations of ΔR = 0.1327 and 0.0865 Å, respectively. © 1993 John Wiley & Sons, Inc.