The conformational equilibria of vanillin and ethylvanillin have been investigated in a supersonic jet expansion using rotational spectroscopy. Two conformers have been detected for each molecule, with a dominant O-H···O intramolecular hydrogen bond locking the local conformation of the hydroxyl and methoxy/ethoxy groups. As a consequence, the observed conformers of vanillin differ only in the orientation of the aldehyde group, either cis or trans with respect to the methoxy group. For ethylvanillin the ethoxy group would plausibly generate additional trans (in-plane) or gauche (out-of-plane) orientations. However, the two detected conformations exhibit only planar ethoxy trans arrangements, with the gauche forms most probably depopulated by collisional relaxation in the jet. Torsional tunneling effects due to internal rotation of the terminal methyl groups were not detectable, indicating internal rotation barriers above 12.3 kJ mol(-1). The conformational population ratios in the jet have been estimated from relative intensity measurements. Ab initio (MP2) and DFT calculations using B3LYP and the recent M05-2X empirical functional supplemented the experimental work, describing the rotational parameters, conformational landscape and the aldehyde and methyl internal rotation barriers in these molecules.