Evidence for internal hydrogen bonding has been sought for in a study of 1,4-butanediol (BDIOL), a molecule in which the "gauche" effect is expected to have minimal influence on the conformational preferences. This has been achieved by studying the molecular structure : and the conformational equilibrium of the BDIOL molecule, using gas-phase electron diffraction (GED). Experimental data have been recorded at 60 keV at two different temperatures, 144 and 260 degrees C. The GED study has been supplemented,d by molecular mechanics calculations, carried out on the ten conformers that are in principle individually distinguishable by the GED method. Refinement of models of mixtures in which the amounts of internal hydrogen-bonded conformers were allowed to adjust led to large increase over those predicted from molecular mechanics in which hydrogen-bonding stabilization was ignored. These results are interpreted as strong evidence for the existence of internal hydrogen bonding. On the basis of the experimental results for the mixture composition, a rough estimate of the energy of the O-H...O-H internal hydrogen bond is 10-11 kJ/mol. Values for the structural parameters and conformational composition are presented.