This Letter reports an MP4(SDQ) ab initio investigation of the electron correlation and basis set effects on the torsional potentials for 2,2'-bipyrrole and 2,2'-bifuran. Pople's standard basis sets and Dunning's double-zeta (D95**) and correlated consistent basis sets (aug-cc-pVDZ) were employed. We also included for the first time thermal corrections to the ab initio relative energies. The torsional potentials were fitted to a truncated Fourier expansion. Our MP4(SDQ)/6-311++G** and MP4(SDQ)/aug-cc-pVDZ improved levels show an agreement within ca. 2 U mol(-1), indicating that convergence has been virtually achieved. Therefore, our new torsional energy data should be used as reference for further studies. (C) 2003 Elsevier Science B.V. All rights reserved.