Summary.The 3,4‐dichloroanilides of four herbicidally active and of four inactive or weakly active phenoxyalkanecarboxylic acids were synthesized and tested for herbicidal activity, in order to study the effect of compounds comprising the two characteristic groupings within one molecule. 4‐Chloro‐2‐methylphenoxyacetic acid, 2,4‐dichloro‐phenoxyacetic acid, 2,4,5‐trichlorophenoxyacetic acid and a‐[2,4,5‐trichlorophenoxy] propionic acid were selected as representatives of active compounds; and phenoxyacetic acid, 2‐methylphenoxyacetic acid, 3,5‐dichlorophenoxyacetic acid and 2,4,6‐trichloro‐phenoxyacetic acid as inactive or weakly active compounds.The compounds were tested in the glasshouse on wheat and white mustard as pre‐and post‐emergence treatments. The synthesized 3,4‐dichloroanilides had no or little herbicidal activity, as compared with the high activity of the known herbicides 3,4‐dichloropropionanilide and 2,4‐dichlorophenoxyacetic acid. The conclusion can be drawn that the introduction of phenoxy groupings into the acid moiety of the active fatty acid‐3,4‐dichloroanilides as well as the introduction of the 3,4‐dichloroaniline group into the carboxylic group of the active phenoxyacetic acids decreases herbicidal activity. This last phenomenon is probably due to a decreased ability to be hydrolysed.L'activité