Competing hydrogen bonding in methoxyphenols: The rotational spectrum of o-vanillin

► The microwave spectrum of o-vanillin was studied in a jet expansion. ► Three different conformations were detected in the spectrum. ► Intramolecular hydrogen bonding controls conformation. ► o-Vanillin prefers OH⋯OC to OH···O hydrogen bonding interactions. ► Supporting ab initio (MP2) and DFT (B3LYP, M05-2X) calculations are provided.