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Benzaldehyde reacts with 2,4,4,5,5-pentamethyl-l,3,2-dioxaphospholane (1) to afford an oxaphosphirane (2) and its dimeric form dioxadiphosphorinane (3) in equilibrium.It is known that benzaldehyde reacts with tricovalent phosphorus derivatives leading to either a 2 : 1 adduct (Scheme 1)1,2 or a 1 : 1 adduct when it is stabilized as a zwitterionic species(I).3 With a-ethylenic ketones, ylides or phosphoranes were obtained4 (Scheme 2).We report here the reaction between benzaldehyde and 2,4,4,5,5-pentamethyl-l,3,2-dioxaphospholane (1) in dichloromethane at room temperature, resulting in a white solid after addition of hexane [m.p. 184°C (decomp.), 60%]. The remaining solution still contains a small amount of (4) and, when the solvent is not well degassed, the oxidation product of (1) (6 31P 40 p.p.m.). The elemental analyses of the new compound correspond to a 1 : 1 adduct.This compound could be oxaphosphirane (2a) or one of its zwitterionic forms (2b) (-'P+-C-Oor (2c) (:P+-0-C-.31P-{lH} n.m.r. spectra show a singlet near -25 p:p.m.,