The mol. and crystal structure of acetylacetone (AA) was studied at 210 and 110 K to reveal the nature of the H bond and possible crystal disorder. The single crystal was grown by an in situ crystn. from the melt using an IR laser beam producing a molten zone in a Lindeman capillary. Crystallog. data and at. coordinates are given. At both temps. the crystals are orthorhombic (space group Pnma, Z = 4) with mols. having a crystallog. mirror plane perpendicular to the mean mol. plane. The intramol. H bond was characterized by the O...O distances 2.541(2) and 2.547(1) .ANG. at 210 and 110 K, with the central H atom equally distributed over the two positions near the oxygens. So, the H bond in AA has two distinct potential min. that are in agreement with previous NMR data and recent quantum-chem. calcns. The bond lengths C-O and C-C in the cis-enol fragment have averaged values (1.291(1) and 1.402(1) .ANG. at 110 K) corresponding to the superposition of two enol isomers due to the crystallog. symmetry. The nature of the crystal disorder (static or dynamic) is discussed using anal. of the at. anisotropic displacement parameters. Charge d. anal. in AA was performed using the high-order x-ray diffraction data at 110 K and a multipole model.