The vibrational dynamics and hydrogen bond topology of excited isomers of carboxylic acid dimers is elucidated by an FTIR study of mixed acetic acid-methyl acetate clusters in supersonic jet expansions. The partial esterification prevents a second OH–O hydrogen bond in the dimer and replaces it by a weak CH–O contact. Vibrational transitions due to mixed acid-ester dimers are observed in the O–H, CO, and C–O stretching range. Similarities between the mixed dimer spectrum and weak bands in the spectrum of pure acetic acid clusters suggest a common hydrogen bond pattern for both species. It is the hydrogen bond pattern observed between two adjacent monomers in solid acetic acid. The conclusions are supported by quantum-chemical calculations.