The rotational spectra of the normal and deuterated species of the acrylic acid-trifluoroacetic acid complex have been assigned for both cis and trans conformations with the pulsed jet Fourier transform microwave technique. Relative intensity measurements suggest that the cis species is more stable than the trans one, in agreement with the ab initio results. The deuteration of the carboxylic groups generates an increase of the distance between the two moieties of the complex (Ubbelohde effect) and prevents the possibility of locating the OH hydrogen atoms. The changes of the Paa planar moment of inertia upon OH → OD substitution allowed for a quantitative estimate of the Ubbelohde effect.