Thiophosphonate compounds having the formula ##STR1## in which R is alkyl, haloalkyl or aryl; X is chloro or SR.sub.2 ; R.sub.1 is alkyl or aryl; and when X is SR.sub.2 then R.sub.2 is a group identical to R.sub.1, or R.sub.1 and R.sub.2 taken together form an optionally alkyl-substituted polymethylene group, are prepared by reaction of a thiophosphonodichloride with a mercaptan in the presence of a catalytic amount of a quaternary ammonium or phosphonium salt, a tertiary amine or hydrohalide thereof or an alkali metal halide combined with a Crown ether, in the substantial absence of a base.