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We measured the molecular beam Fourier transform microwave spectra of four isotopic species of of the dimer of \textit{tert}-butanol, that is C$\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}$H$\mathrm{<mrow\; data-mjx-texclass="ORD">9</mrow>}$-OH$\cdots$O(H)-C$\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}$H$\mathrm{<mrow\; data-mjx-texclass="ORD">9</mrow>}$, C$\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}$H$\mathrm{<mrow\; data-mjx-texclass="ORD">9</mrow>}$-OH$\cdots$O(D)-C$\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}$H$\mathrm{<mrow\; data-mjx-texclass="ORD">9</mrow>}$, C$\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}$H$\mathrm{<mrow\; data-mjx-texclass="ORD">9</mrow>}$-OD$\cdots$O(H)-C$\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}$H$\mathrm{<mrow\; data-mjx-texclass="ORD">9</mrow>}$, and C$\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}$H$\mathrm{<mrow\; data-mjx-texclass="ORD">9</mrow>}$-OD$\cdots$O(D)-C$\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}$H$\mathrm{<mrow\; data-mjx-texclass="ORD">9</mrow>}$. We observed that the H -- D isotopic substitution of the hydroxylic hydrogen participating in the O-H$\cdots$O Hydrogen bond in the \textit{tert}-butanol dimer produces an increase of the B and C rotational constants, according to the shrinkage of the O$\cdots$O distance, underlying and sizing the associated Ubbelhode effect. , 1955, 8, 71} The conformation and structure of the complex, and an estimation of the Ubbelhode effect have been obtained by combining the experimental data with the results of MP2/6-311++G** ab initio calculations.