Pesticidal dicyanohydrin (thiono)(dithio)-phosphates(phosphonates) of the general formula ##STR1## in which X represents oxygen or sulphur,R represents an optionally substituted radical selected from alkyl, alkenyl, alkinyl, aryl, aralkyl, alkoxy, aryloxy, aralkoxy, alkylthio, arylthio, aralkylthio, alkylamino (mono- and dialkylamino), arylamino and aralkylamino, andR.sup.1 and R.sup.2 are identical or different and individually represent a hydrogen atom, an optionally substituted radical selected from alkyl, alkenyl, alkinyl, cycloalkyl, cycloalkenyl, aralkyl, aralkenyl and aryl, or an optionally substituted heterocyclic radical,are new and are particularly effective in combating pests, such as insects and acarids, in synergistic combinations with other compounds which are active against arthropods.