The ground state conformations and transition states for conformer interconversion in the two isomers of the of furan (1 and 2) and thiophene (3 and 4) were identified through a theoretical approach based on ab-initio molecular orbital calculations. Two minima and two transition states were found in the potential energy profiles for all the molecules, but the behaviour of 2-methylsulphinyl furan (1) differs somewhat from that of the other compounds. The minima correspond to the two orientations of the S-O bond with respect to the heterocyclic heteroatom X (X = O or S), labelled X,O-cis an X,O-trans. The conformer having the S-O bond oriented s-cis with respect to the internal double bond with higher pi density (X,O-cis for the 3-methylsulphinyl derivatives and X,O-trans for the 2-methylsulphinyl derivatives) has a twist angle phi (between the S-O bond and the heterocyclic ring) of a few degrees. Larger distortions are present in the conformers with opposite orientation of the S-O bond, and the twist angle has the highest value (phi = 62.8-degrees) in compound 1. The transition states correspond to the methyl group being nearly coplanar with the heterocyclic ring and this situation seems to be due mostly to a loss of pi electron delocalization. More complex is the transition state situation for compound 1, since a barrier in the potential energy for internal rotation is imposed also by the repulsion between the heterocyclic and sulphinyl oxygen atoms. The conformer populations were estimated from the potential energy