A process for the preparation of 2,4-dichloro- or 2-methyl-4-chlorophenoxyacetic acid or alpha -2,4-dichloro- or alpha -2-methyl-4-chlorophenoxypropionic acid by reacting phenoxyacetic or 2-methylphenoxyacetic acid or alpha -phenoxypropionic or alpha -2-methylphenoxypropionic acid with chlorine in water, wherein phenoxyacetic acid or phenoxypropionic acid is precipitated from an aqueous solution thereof, chlorine gas is passed into the resulting suspension of phenoxyacetic or 2-methylphenoxyacetic acid or alpha -phenoxypropionic or alpha -2-methylphenoxypropionic acid in water at from 30 DEG to 70 DEG C. at a rate not exceeding the rate at which it is consumed in the suspension, until from 100 to 110% of the theoretically required amount of chlorine gas has been taken up by the suspension, and the 2,4-dichloro- or 2-methyl-4-chloro-phenoxyacetic acid or alpha -2,4-dichloro- or alpha -2-methyl-4-chloro-phenoxypropionic acid is isolated from the suspension.