The gauche effect, the phenomenon of approximating (dihedral angle about 60 degrees) electronegative atoms or groups in a 1,2-disubstituted ethane fragment, is theoretically evaluated in this report, taking 1,2-difluorocyclopropane and 1,2-difluorocyclohexane (cis and trans) as models. NBO analysis was used in order to give insight into the nature of the gauche effect in the studied compounds, i.e. its origin has been evaluated as due to hyperconjugation or to bent bonds. (c) 2007 Elsevier B.V. All rights reserved.80741699159162