The structure of 4,7-dioxononanoic acid previously assigned on the basis of its NMR spectra, was confirmed by X-ray crystallography. The compound exhibits only one sharp peak in its solid state IR spectrum at 1685 cm -1 for its three carbonyl groups. A plausible explanation for this anomaly is provided by X-ray crystallography on the basis of crystal packing and dipole-dipole interactions in aligned arrays of carbonyl groups. This alignment is found in the carboxylic acid group as well, so that the hydrogen bonded interactions of the two carboxylic acid groups found normally in the "dimeric" structures of carboxylic acids in the solid state are not present in the title compound.