Abstract Microwave spectra have been measured for various vibrationally excited states of the OCS molecule. The l-type doubling constants obtained for the excited states of the bending vibration, ν2, have led to the determination of the zero-order coupling constant, qe. It has been shown that the qe value thus obtained is useful for the evaluation of the harmonic force constants. The Fermi resonances have been discussed; they lead to the conclusion that the effect due to the cubic constant, k223, cannot be ignored, nor can that due to k122 With due consideration for this situation, the equilibrium structure has been found to be: re(O–C)=1.1572±0.002 Å and re(C–S)=1.5606±0.002 Å.