Geometry-relaxed ab initio calculations of acetic anhydride at interalia B3LYP/6-31G**, B3LYP/cc-pvtz, and MP2/6-31G** level revealed a mixture of nonplanar (sp,sp) and (sp,ac) energy minima, connected to one another via low-energy rotation barriers, thereby allowing for extensive large-amplitude motions. This model provided the geometrical constraints and force fields necessary to perform the joint analysis of gasphase electron diffraction and infrared data. The large-amplitude motion is described, using pseudoconformers at 20 degrees intervals around the axes of rotation. It led to a dynamic model consisting of eight pseudoconformers of lowest energy connected to the two local minima ((sp,ac) and nonplanar (sp,sp)) by fixed differences in torsion angles. The main structural parameters were refined using the electron diffraction method and an experimental "conformer" ratio of nonplanar (sp,sp)/(sp,ac) = 37(+/-15)%:63(+/-15)% was obtained, in close agreement with the quantum chemical results. The model of acetic anhydride is self-consistent, reproduces the IR frequencies, with a root-mean-square deviation of about 10 cm(-1). and results in an improved frequency assignment. Assisted by MP2/6-31G**-based IR band intensities, the model also explains the following experimental spectral peculiarities: (i) the relatively large number of bands with a small intensity and (ii) the changes in band intensities, band shape and doubler behavior when going from the gas phase to the liquid and to solutions of different polarity.