Abstract The quadratic, cubic and semi-diagonal quartic force field of ethyl cyanide has been calculated at the B3LYP level of theory employing a basis set of triple-ζ quality. A semi-experimental equilibrium structure has been derived from experimental ground state rotational constants and rovibrational interaction parameters calculated from the ab initio force field. This structure is in excellent agreement with the ab initio structure calculated at the CCSD(T) level of theory using a basis set of quadruple-ζ quality and a core correlation correction. The empirical structures are also determined and their accuracy is discussed. The potential barrier V 3 hindering internal rotation of the methyl group has been calculated from 23 rotational transitions of CH 3CH 2C 15N which were found split into doublets, giving V 3 = 3074(27) cal mol −1.