The molecular structure of 1,3-dihydroxyacetone (DHA) has been studied by gas-phase electron diffraction (GED), combined analysis of GED and microwave (MW) data, ab initio, and density functional theory calculations. The equilibrium r(e) structure of DHA was determined by a joint analysis of the GED data and rotational constants taken from the literature. The anharmonic vibrational corrections to the internuclear distances (r(e) - r(a)) and to the rotational constants (B-e((i)) -B-0((i))) needed for the estimation of the r(e) structure were calculated from the B3LYP/cc-pVTZ cubic force field. It was found that the experimental data are well reproduced by assuming that DHA consists of a mixture of three conformers. The most stable conformer of C-2 upsilon symmetry has two hydrogen bonds, whereas the next two lowest energy conformers (C-s and C-1 symmetry) have one hydrogen bond and their abundance is about 30% in total. A combined analysis of GED and MW data led to the following equilibrium structural parameters (r(e)) of the most abundant conformer of DHA (the uncertainties in parentheses are 3 times the standard deviations): r(CO) = 1.215(2) angstrom, r(C-C) = 1.516(2) angstrom, r(C-O) = 1.393(2) angstrom, r(C-H) = 1.096(4) angstrom, r(O-H) = 0.967(4) angstrom, angle C-CO = 119.9(2)degrees, angle C-C-O = 111.0(2)degrees, angle C-C-H = 108.2(7)degrees, angle C-O-H = 106.5(7)degrees. These structural parameters reproduce the experimental B-o((i)) values within 0.05 MHz. The experimental structural parameters are in good agreement with those obtained from theoretical calculations. Ideal gas thermodynamic functions (S degrees(T), C-p degrees(T), and H degrees(T) - H degrees(0)) of DHA were calculated on the basis of experimental and theoretical molecular parameters obtained in this work. The enthalpy of formation of DHA, -523 +/- 4 kJ/mol, was calculated by the atomization procedure using the G3X method.