The molecular geometries of benzaldehyde and salicylaldehyde have been determined by gas-phase electron diffraction and ab initio molecular orbital calculations at the MP2(FC)/6-31G* level. Several parameter differences from the molecular orbital calculations were incorporated as constraints in the electron diffraction analysis of salicylaldehyde. Some selected bond lengths (r(g)) and angles obtained in the electron diffraction analyses are as follows : benzaldehyde (C-H)(mean) 1.095 +/- 0.005 Angstrom; (C-C)(mean) (benzene) 1.397 +/- 0.003 Angstrom; C-2-C-7 1.479 +/- 0.004 Angstrom; C=O 1.212 +/- 0.003 Angstrom; C-2-C-7=O 123.6 +/- 0.4 degrees; the benzene ring is undistorted within experimental error; salicylaldehyde (C-H)(mean) 1.090 +/- 0.011 Angstrom, (C-C)(mean) (benzene) 1.404 +/- 0.003 Angstrom; C-1-C-2 1.418 +/- 0.014 Angstrom; C-O 1.362 +/- 0.010 Angstrom; O-H 0.985 +/- 0.014 Angstrom; C-2-C-13 1.462 +/- 0.011 Angstrom; C=O 1.225 +/- 0.004 Angstrom; C-2-C-13=O 123.8 +/- 1.2 degrees; C-2-C-1-O 120.9 +/- 1.1 degrees. All the data are consistent with planar equilibrium structures for both molecules. The barrier to formyl group torsion is estimated to be appreciably higher for salicylaldehyde (at least 30 kJ/mol) than for benzaldehyde (at least 20 kJ/mol). There is intramolecular hydrogen bonding in the salicylaldehyde molecule of comparable strength with that in o-nitrophenol. The hydrogen bond is characterized by the following observed/calculated distances : O ... H-(-O) 1.74(2)/1.80 Angstrom and O ... O 2.65(1)/2.68 Angstrom. The structural changes in the rest of the molecule, as compared with the parent benzaldehyde and phenol molecules, are consistent with resonance-assisted hydrogen bonding similar to the o-nitrophenols. These changes include a lengthening of the C=O bond (0.013 Angstrom), a shortening of the exocyclic C-C bond (0.020 Angstrom), a lengthening of the ring C-C bond between the substituents (0.017 Angstrom), and a shortening of the hydroxy C-O bond (0.022 Angstrom).