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We present an investigation of the deuterium nuclear quadrupole coupling in a series of carbonic acids R-COOH: Among those, the acids with the groups $R=CH3$ and $R=CF3$ are two extreme cases for substituent effects on the charge distribution at the carboxyl group. The deuterium nuclear quadrupole coupling serves as a sensitive probe for the electric field gradient at the location of the acidic proton. The rotational spectra and the quadrupole coupling hyperfine structures of several R-substituted carbonic acids $R-COO2H$ in the range of 6 to 26.5 GHz are reported. Quantum chemical calculations were performed to determine the nuclear quadrupole coupling tensor of $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}H$. The analysis of the nuclear quadrupole coupling in the rotational spectra provided experimental information on the tensors.