The structure of formic acetic anhydride was studied by the joint analysis of gas-phase electron diffraction and infrared data, supported with extensive ab-initio calculations on the 4-21G and 6-31G** levels. All data agree with the gas phase at room temperature existing in the planar (sp,ap) conformer. Best electron diffraction geometry was:obtained using geometrical constraints derived from 4-21G calculations after correction to r(alpha)(o) level. Also, the scaled 4-21G force field performed better than its 6-31G** counterpart. The new model of formic, acetic anhydride is self-consistent, reproduces the IR frequencies with a root-mean-square deviation of 8.8 cm(-1), and results in an improved frequency assignment as well as in a good qualitative agreement between observed and calculated IR band intensities. Formic acetic anhydride is conformationally and spectroscopically very different from acetic anhydride,but strongly resembles formic anhydride, although differences remain. The similarities with formic anhydride are ascribed to an attractive nonbonded H (formyl)... O= interaction, while the dissimilarities are ascribed to a larger electronic interaction between two formyl moieties rather than between a formyl and an acetyl moiety.