The centrifugal distortion constant DK of a symmetric top molecule has proved a very elusive parameter to estimate by spectroscopic analysis. Values of DK have been calculated for the molecules BF 3, SO 3, NH 3, PH 3, AsH 3, NF 3, PF 3, AsF 3, CH 3D, SiH 3D, GeH 3D, CH 3F, CH 3Cl, CH 3Br, CH 3I, CH eCN, CH 3CCH, H 2C 3H 2, cyclo-C 3H 6, and isotopic modifications thereof, in terms of their quadratic force constants. In all cases the force constants used are reliably determined by existing spectroscopic data. Where direct comparison is possible, good agreement with experimental DK values is found, and the calculated constants are considered reliable to ∼± 5%. The constants are intended to be of assistance in future spectroscopic and structural studies on the molecules listed. Structural applications to methyl iodide, silane, and cyclopropane are considered.