Characterization of protonated formamide clusters by vibrational predissociation spectroscopy confirms theoretical predictions that O-protonation occurs in preference to N-protonation in formamide. The confirmation is made from a close comparison of the infrared spectra of H+[HC(O)NH2](3) and NH4+[HC(O)NH2](3) produced by a supersonic expansion with the spectra produced by ab initio calculations. Fur NH4+[HC(O)NH2](3), prominent and well-resolved vibrational features are observed at 3436 and 3554 cm(-1). They derive, respectively, from the symmetric and asymmetric NH2 stretching motions of the three formamide molecules linked separately to the NH4+ ion core via three N-H+. . .O hydrogen bonds. Similarly distinct absorption features are also found for H+[HC(O)NH2](3); moreover, they differ in frequency from the corresponding vibrational modes of NH4+[HC(O)NH2](3) by less than 10 cm(-1). The result is consistent with a picture of proton attachment to the oxygen atom, rather than the nitrogen atom in H+[HC(O)NH2](3). We provide in this work both spectroscopic and computational evidence for the O-protonation of formamide and its clusters in the gas phase.