The rm method of determining molecular structures from isotopic zero-point rotational constants is tested by applying it to synthetic data for OCS calculated from an assumed equilibrium structure and force field. The results show that the rm structure is significantly different from the re structure, unless the substitution coordinates used in the rm calculation are corrected for the use of finite changes of mass ∆ mi by extrapolation to ∆ mi = 0. The latter procedure is not usually feasible experimentally. In the ordinary rm method the errors are generally comparable in magnitude to those in Costain's rg method. An exception arises when the number of structural parameters equals the number of independent moments of inertia of a single isotope (e.g., a diatomic molecule or a symmetrical XY 2 molecule), in which case the rm structure is in good agreement wich the re structure.