Abstract: The present invention relates to a new process for the preparation of phosphonic acid esters of the formula I ##STR1## in which Ar denotes phenyl, a binuclear or polynuclear aromatic radical, cycloheptatrienyl, furyl or pyridyl, X denotes (substituted) alkyl, (substituted) alkoxy or optionally substituted aryl, aryloxy, heterocyclyloxy or arylmercapto radicals, phenylcarbonylamino, alkylcarbonylamino or alkoxycarbonylamino, alkylmercapto, alkoxycarbonyloxy, alkoxycarbonylalkoxy, phenylaminocarbonyloxy or alkylaminocarbonyloxy or two radicals X in the orthoposition together form a (substituted) (C.sub.3 -C.sub.4) chain in which 2 carbon atoms can be replaced by oxygen, Y denotes halogen, alkoxycarbonyl, alkylcarbonyl, nitro, nitroso, cyano, alkylsulfonyl, phenylsulfonyl, alkoxysulfonyl, phenoxysulfonyl, alkoxy/alkylphosphinyl, halogenated alkyl, formyl or (substituted) amidosulfonyl, R denotes alkyl, n denotes 1, 2 or 3 and m denotes 0, 1, 2 or 3, wherein a compound of the formula IIX.sub.n --Ar--Y.sub.