Halotrimethylsilanes Me(3)SiX (X = Br, I) catalyse rearrangements of tricoordinate phosphorus esters R'R''P-OR into the corresponding phosphoryl systems R'R''P(O)R. This provides a simple and efficient route to a variety of structures containing phosphorus-carbon bonds, under mild conditions and with good yields. The reaction mechanism was investigated in detail by (31)P NMR spectroscopy and independent synthesis of the reaction intermediates. It has been demonstrated that the primary products of this catalytic reaction are halogeno P(III) structures R'R''PX and silyl ethers ROSiMe(3) and that they subsequently react to give the corresponding phosphorus silyl esters-Me(3)SiOPR'R''-and alkyl halides RX. At higher temperatures these intermediates then react to form R'R''P(O)R compounds. This paper also features the surprising observation that when esters Ph(2)POR and halotrimethylsilanes Me(3)SiX (X = Br, I) are used in 2:1 ratio, phosphonium salts Ph(2)R(2)P(+)X(-) and trimethylsilyl diphenylphosphinate--Ph(2)P(O)OSiMe(3)--are formed as the major products. Experimental evidence indicates that the mechanisms of both reactions are fundamentally different from that of the Michaelis-Arbuzov reaction. Me(3)SiCl is not reactive and this paper explains why.