Trimesityltin bromide reacts with concentrated aqueous ammonia to give trimesityltin hydroxide. This compound eliminates mesitylen and yields dimesityltin oxide. The product was investigated bv 13C NMR, mass spectroscopy and X-ray diffractions. Di-mesityltinoxide crystallizes in the space group P21/w with a = 25.38(1) Å, b = 12.525(5) Å, c= 32.91(1) Å, β= 91.37(3)° and two molecules in the asymmetric unit (Z=8). The R-factor is 0.049 for 4688 symmetry-independent reflections. The trimeric molecules have six-membered tin-oxygen heterocycles in a twist conformation.