By comparison of i.r. intensity measurements and microwave Stark effect information, it is established experimentally that the absolute signs of the dipole moment derivative for carbonyl stretching in OCS is negative; that is ∂p/∂r_CO=–1·5 ee_CO where e is the charge on the proton and e_CO is a unit vector from the carbon towards the oxygen atom, or, in more familiar, non-SI units the dipole moment gradient with respect to CO stretch is 7 D/Å with the oxygen becoming more negative.