The molecular geometry of 4,6-dinitroresorcinol has been determined by gas-phase electron diffraction. The resonance-assisted intramolecular hydrogen bonding is accompanied by pronounced bond length changes as compared with the parent molecules phenol and nitrobenzene. The molecular geometry is characterized by the following bond lengths (r(g)) and bond angles : (C-H)(mean), 1.086 +/- 0.015 Angstrom; (C-C)(mean), 1.402 +/- 0.003 Angstrom; C-3-C-4, 1.424 +/- 0.005 Angstrom; C-O, 1.341 +/- 0.004 Angstrom; O-H, 1.002 +/- 0.009 Angstrom; C-N, 1.461 +/- 0.004 Angstrom; (N-O)(mean), 1.233 +/- 0.003 Angstrom; angle C-1-C-2-C-3, 120.3 +/- 0.4 degrees; angle C-2-C-3-C-4, 119.3 +/- 0.3 degrees; angle C-3-C-4-C-5, 121.6 +/- 0.3 degrees; angle C-4-C-5-C-6, 118.0 +/- 0.4 degrees; angle C-4-C-3-O, 123.4 +/- 0.3 degrees; angle C-O-H, 104.5 +/- 1.4 degrees; angle C-3-C-4-N, 121.0 +/- 0.2 degrees; (angle C-N-O)(mean), 118.1 +/- 0.1 degrees; angle O-N-O, 123.7 +/- 0.2 degrees. The effective angle of nitro group torsion from electron diffraction is 14.4 +/- 1.3 degrees. If it is assumed that this effective angle of torsion emerges as a consequence of torsional vibrations about the planar equilibrium conformation, then a barrier to internal rotation of 18 +/- 3 kJ/mol may be estimated, the same as in nitrobenzene, the planarity of which is consistent with microwave spectroscopic results.