Diphenyl ether derivatives of formula II ##STR1## wherein R.sub.1 represents a hydrogen or halogen atom or an alkyl or haloalkyl group, R.sub.2 and R.sub.3, which may be the same or different, each independently represents a hydrogen or halogen atom, or an alkyl, haloalkyl, nitro or cyano group, R.sub.4 represents an alkyl group and X represents an oxygen or sulphur atom, useful as intermediates in the preparation of diphenyl ether herbicides, are prepared by treating a compound of formula III ##STR2## where R.sub.1, R.sub.2, R.sub.3 and X are as defined above and R.sub.6 is a hydrogen atom or an alkyl group, with an alkali under hydrolyzing conditions.