The geometric structure and conformational properties of 3,3,6,6-tetramethyl-1,2,4,5-tetroxane (diacetone diperoxide) have been studied by gas electron diffraction and quantum chemical calculations (MP2 and B3LYP methods with 6-31G(d,p) and 6-311+G(2df,p) basis sets). The molecule possesses a chair conformation with C-2h symmetry and the following geometric parameters for the six-membered ring (r(h1) values) have been determined: O-O = 1.463(5) A, C-O = 1.432 (4) A, O-C-O = 108.2(7)degrees, C-O-O = 107.7(4)degrees, phi(C-O-O-C) = 63.7(4)degrees, and phi(O-O-C-O) = -63.9(4)degrees. A small contribution of less than 3.5% of a twist conformer with C-2 symmetry cannot be excluded. Quantum chemical calculations predict a contribution between 1 and 2%. Additional calculations for the parent compound 1,2,4,5-tetroxane (diformaldehyde diperoxide) demonstrate that methyl substitution at the carbon atoms has a minor effect on the ring geometry but a strong effect on the conformational properties. Methyl substitution reduces the energy difference between twist and chair conformers by more than 5 kcal/mol.