Structure of the ortho-Cl-substituted biphenyls is studied in vacuo and in the CCl4 solution, by molecular modelling methods. Ab initio determinations of rotational potentials are followed by molecular dynamics simulations, in order to estimate effects of the solvent on conformations. The solvent flattening effect is correlated to molecular volume changes during rotation around the C-C bond. It is concluded, that solvent induced change of rotational barrier can have influence on molecular properties of studied compounds, especially their toxicity. (C) 2003 Elsevier Science B.V. All rights reserved.