Electron diffraction experiments on the vapor of 1,4-cyclohexanedione have been carried out at a nominal temperature of 435 K. The results are consistent with the presence of a mixture of a chair form of C-2h symmetry and a twisted boat form of D-2 symmetry. The former has the familiar dynamic properties of a semirigid molecule, but the D-2 form undergoes a large-amplitude twisting motion (pseudorotation) that degrades the symmetry to C-2. The analysis was designed to elucidate parameter values and internal dynamics of each conformer and the composition of the system. The large-amplitude motion of the twisted boat form was modeled by placement of 10 pseudoconformers at approximately 5 degrees intervals along a pseudorotational coordinate that began at the D-2 position and that reflected the angle between the C=O bond vectors. A Gaussian weighting of the pseudeoconformers centered on the (lowest-energy) D-2 position was assumed. Differences in the interatomic distances and bond angles of these pseudoconformers were calculated via B3LYP/cc-pVTZ theory and introduced as constraints. The bond length averages over the twisted boat forms followed by values for the chair in square brackets are (r(g)/angstrom; angle(alpha)/deg) r(C-H) = 1.115(11) [1.124(11)], r(C=O) = 1.211(3) [1.233(6)], r(C1-C2) = 1.524(5) [1.526(5)], and r(C2-C3) = 1.533(11) [1.539(11)]. The corresponding ring angle values are angle(C1C2C3) = 111.1(5) [111.0(4)] and angle(C6C1C2) = 116.3(8) [115.7(8)]. In the twisted boat form, pseudorotation leads to a weighted average displacement of the angle between the C=O bond vectors, angle Delta(CO,CO), equal to 21.3 degrees from the 180 degrees value in the D-2 form corresponding to an average angle between the CO bond vectors of 158.7(1)degrees. The amount of the chair form in the gas at 435 K is 24(10)%. The listed uncertainties are estimated at 2 sigma.