A cyanohydrin phosphate of the formula ##STR1## wherein R represents a hydrogen atom or an optionally substituted radical selected from alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkenyl, aralkyl, aralkenyl and aryl, or an optionally substituted heterocyclic radical,R.sup.1 and R.sup.2 are identical or different and individually represent optionally substituted radicals selected from alkyl, alkenyl, alkynyl, aryl, aralkyl, alkoxy, aryloxy, aralkoxy, alkylthio, alkenylthio, alkinylthio, arylthio, aralkylthio, alkylamino (monoalkylamino or dialkylamino), arylamino and aralkylamino, or together represent alkanediyl, alkanedioxy, aminoalkyloxy or alkanediamino, andX represents oxygen or sulphur,are obtained by the reaction of a phosphoric acid chloride of the general formula ##STR2## wherein X, R.sup.1 and R.sup.2 have the meaning given above, with an aldehyde of the general formulaR--CHO (III)wherein R has the meaning given above, in the presence of an approximately equimolar quantity of water-soluble cyanides. Some of the compounds of the formula (I) are new. They can be used as insectides and acaricides.