The structure of formic anhydride was studied by the joint analysis of gas-phase electron diffraction, microwave, and infrared data. The experimental data are supported with geometrical constraints and force fields from geometry-relaxed ab-initio calculations on the 4-21G and 6-31G** levels. All data agree with the gas phase at room temperature consisting of the planar [sp,ap] conformer, in which the two O=CH-O moieties differ significantly in geometrical as well as vibrational parameters. Geometrical least-squares constraints taken from the 4-21G calculations performed slightly better than those from 6-31G** calculations. In contrast, the 6-31G**-derived scaled force field performed better in the IR analysis, reproducing the frequencies with a root-mean-square deviation of 6.7 cm(-1) and a largest discrepancy of 14.1 cm(-1). Assisted by 6-31G**-based IR band intensities, a significantly improved assignment of IR frequencies was made. The reported self-consistent molecular model of formic anhydride is in agreement with all diffraction and spectroscopic data available to date, contains a complete force field of which diagonal and off-diagonal constants allow a physically consistent interpretation, and also points to a rationalization why formic anhydride is thermolabile and acetic anhydride is not.