Ground state rotational constants were determined for 14 isotopologues of o-xylene. These rotational constants have been corrected with the rovibrational constants calculated from a quantum chemical force field. It was found that the derived semiexperimental equilibrium rotational constants of the deuterated isotopologues are not fully compatible with those of the non-deuterated ones. To mitigate the consequences of this incompatibility, the semiexperimental equilibrium rotational constants of the non-deuterated isotopologues have been supplemented by structural parameters, in particular those for hydrogen atoms, from high level ab initio calculations. The combined data have been used in a weighted least-squares fit to determine an accurate equilibrium structure. It was shown, at least in the present case, that the empirical structures are not sufficiently accurate and are, therefore, hardly appropriate for large molecules with many hydrogen atoms.