Results from a single crystal X-ray diffraction study on o-phthalaldehyde (1) revealed dihedral angles between the CHO groups and the benzene ring of 13(2) o and 27(2) o . In acetaldehyde solution, the 1 H-NMR of 1 showed only broadening of the aldehyde singlet down to -120 o C. The microwave spectrum of 1 was analyzed to give a non-zero second-moment P c c of 0.662u-A 2 , consistent with a non-planar structure in the gas phase with the CHO groups rotated out-of-plane by an average of 10+/-2 o . The X-ray determined structure, microwave experimental results and ab initio calculations at the MP2/6-31+G(d) level all agree that 1 has a nonplanar structure. There is a remarkable agreement between the structure derived from X-ray and ab initio methods with electron correlation. Ab initio calculations at the Hartree-Fock level optimized to planar structures.