The preparative scale synthesis of dimesityldioxirane (5) via oxidn. of dimesitylcarbene (6) is described. The dioxirane was obtained as colorless cryst. material, completely stable at -20 DegC and characterized by spectroscopic methods, X-ray crystallog., and DFT calcns. (B3LYP/6-31G(d)). The mol. shows approx. local C2 symmetry with the mesityl rings twisted by ca. 60 Deg. The exptl. structural parameters, e.g., the O-O distance of 1.503 .ANG., are in good agreement with the theor. 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 soln. are nicely reflected by measured and calcd. (SOS-DFPT) NMR chem. shifts.