Upon supersonic expansion, formic acid and cyclobutanone (CBU) form a molecular cluster in which the two constituent molecules, linked by OHO and CHO hydrogen bonds, undergo a rapid interconversion between two equivalent forms. The tunneling motion takes place through the rupture and reformation of the C-HO hydrogen bond between the carbonyl oxygen of HCOOH and one of the two hydrogen atoms of the methylenic group adjacent to the cyclobutanone keto group. From the microwave spectra, tunneling energy splittings (ΔE01) have been determined for the parent (1122.756(3) MHz), DCOOHCBU (1084.538(1) MHz) and HCOODCBU (1180.282(4) MHz) isotopic species. From these splittings, the potential barrier to interconversion has been calculated to be B2 = 39.7(5) cm-1. The tunneling pathway is an asymmetric butterfly-like motion between the two moieties of the adduct, with a barrier at a configuration in which the ring plane of cyclobutanone is coplanar with formic acid.