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Microwave and millimeter-wave rotational spectra of 46 thiophene isotopologues are used to determine a new semi-experimental equilibrium (reSE) structure for thiophene. The large dataset of isotopologues—roughly twice as large as any dataset used for a semi-experimental equilibrium structure determination for a molecule of this size—and the broad frequency coverage enable a detailed investigation concerning previously reported discrepancies between the structural parameters of the reSE and re structures of thiophene. The observation of rotational spectra for 23 new thiophene-dx isotopologues, many with additional 34S or 13C substitution at natural abundance, is made possible by the high sensitivity of the PARIS FTMW spectrometer in the microwave frequency region. Frequency coverage for thiophene and single heavy-atom isotopologues in the millimeter-wave region has been expanded to include the range of 82–750 GHz. Bond distances and bond angles of the new highly precise and accurate reSE structure are determined with precisions of ≤0.0002 Å and ≤0.02°, respectively. This study provides strong evidence that the discrepancies can be addressed only by improvements to the theoretical re structure and that the precision and accuracy of the semi-experimental equilibrium (reSE) structural parameters are approaching the limit of structure determination.