Prepd by passing chlorine into diethylphosphorous acid in ligroin contg Na wire: Arbusow, Arbusow, Ber. 65, 195 (1932); by heating triethyl phosphite and phosphorus trichloride; or by adding phosphorus trichloride to a soln of ethanol and diethylaniline in ether: Cook et al., J. Chem. Soc. 1949, 2021.