Several stable dimeric dialkylaluminum boryloxides of the formula [(mu -Mes(2)BO)AlR2](2) (R = Me (1), Et (2),Bu-1) have been prepared from dimesitylborinic acid Mes(2)BOH and trialkylaluminums R3Al. Compound 1 has been characterized by X-ray diffraction. These compounds exhibit diverse reactivity toward protonolytic reagents depending on the bulkiness of these reagents. Treatment of 1 with tert-butyl alcohol afforded crystalline species trans-[(mu-(BuO)-Bu-t)(Mes(2)BO)AlMe](2) (3), which is the first example of a mixed system containing boryloxide and alkoxide ligands together. Most surprisingly, Mes(2)BOH was found to undergo catalytic decomposition in the presence of [(mu -Mes(2)BO)AlR2](2) via the unexpected cleavage of one boron-carbon bond. The molecular structure of the decomposition product, i.e., trimesitylboroxin [MesBO](3) (4) is reported.