Ab initio energy evaluations on the Z and E rotamers, as well as on the rotational transition states connecting them, were carried out for methyl formate and trifluoromethyl formate. The estimates for the energy differences at the basis set limit suggested that the Z/E energy difference and the rotational barrier height are both significantly smaller for trifluoromethyl formate than for methyl formate. Our best estimate for the enthalpy difference at 298 K between the Z and E rotamers for methyl formate is 5.21 kcal/mol, which is slightly larger than the upper edge of uncertainty of previously reported experimental value. (C) 2003 Elsevier Science B.V. All rights reserved.