Rotational spectra for the normal and four isotopically substituted species of the 1: 1 complex between methyl fluoride (H3CF) and carbonyl sulfide (OCS) have been measured using Fourier-transform microwave spectroscopy in the 5-16 GHz frequency region. The observed spectra fit well to a semirigid Watson Hamiltonian, and an analysis of the rotational constants has allowed a structure to be determined for this complex. The dipole moment vectors of the H3CF and OCS monomers are aligned approximately antiparallel with a C center dot center dot center dot C separation of 3.75(3) angstrom and with an ab plane of symmetry. The values of the P-cc planar moments were found to be considerably different from the expected rigid values for all isotopologues. An estimate of similar to 14.5 (50) cm(-1) for the internal rotation barrier of the CH3 group with respect to the framework of the complex has been made using the P-cc values for the H3CF-OCS and D3CF-OCS isotopic species. Two structures, very close in energy and approximately related by a 60 degrees rotation about the C-3 axis of the methyl fluoride, were identified by ab initio calculations at the MP2/6-311++G(2d,2p) level and provide reasonable agreement with the experimental rotational constants and dipole moment components.