The preparative scale synthesis of dimesityldioxirane (5) via oxidation of dimesitylcarbene (6) is described. The dioxirane was obtained as colorless crystalline material, completely stable at -20 degrees C and characterized by spectroscopic methods, X-ray crystallography, and DFT calculations (B3LYP/6-31G(d)). The molecule shows approximately local C-2 symmetry with the mesityl rings twisted by ca. 60 degrees. The experimental structural parameters, e.g., the O-O distance of 1.503 Angstrom, are in good agreement with the theoretical values. The relief of steric strain caused by the mesityl groups is discussed in terms of the geometric distortion of the three-membered ring and the mesityl groups. Conformational features of 5 in solution are nicely reflected by measured and calculated (SOS-DFPT) NMR chemical shifts.