Abstract. The carbodiphosphorane C(PPh 3 ) 2 (1) is easily hydrolyzed from wet air to give the ylide Ph 3 PCHP(O)Ph 2 (2), which forms addition compounds with various Lewis acids to give neutral or cationic compounds. According to pairs of electrons at the central carbon atom and the oxygen atom, respectively, addition compounds with coordination modes A (via oxygen), B (via carbon), and C (via carbon and oxygen) were isolated either as by-products from reactions of Lewis acids with 1 (contaminated with some 2) or directly with 2. The crystal structures and the spectroscopic properties of 2 and of the addition