The barrier heights to methyl group rotation in two dimethylnaphthalene isomers have been studied by a combination of variable temperature neutron diffraction and ab initio calculations of the PES. The materials show differing degrees of steric hindrance of the torsional motion of the methyl groups, resulting in substantially different barrier heights. The relative heights of the barrier to rotation found from a simple harmonic interpretation of the experimental neutron data are reproduced well in the calculations. Moreover by using fairly extended basis sets (at the 6-31 G** level) reasonable quantitative agreement between the experimental and computed barrier heights are found. The PES calculations have allowed the investigation of possible coupled motions of the methyl groups, not accessible to diffraction measurements. (C) 2002 Elsevier Science B.V. All rights reserved.