Hydrolyses of trimesitylgallium and -aluminum were monitored by H-1 NMR spectroscopy in the temperature range -60 OC to room temperature using deuterated THF as solvent. The first intermediate of the reaction using water and trimesitylgallium is the monomeric water adduct, (Mes(3)Ga . OH2).2THF, 1. Elimination of mesitylene leads to the dimeric hydroxide (Mes(2)GaOH)(2) . THF, 2. Excess of water gives Mes(6)Ga(6)O(4)(OH)(4) . 4THF, 3. As a result of this reaction course the corresponding hydrolysis of trimesitylaluminum leads to (Mes(3)Al . OH2). nTHF, 4 and (Mes(2)AlOH)(2) . 2THF, 5. The molecular structures of compounds 1, 2, and 5 have been determined by low temperature X-ray structure analysis. The Lewis acidities of the mesityl containing hydroxides are discussed and compared with those of known aryl- and alkyl aluminum and gallium hydroxides.