The invention relates to a method for preparing 2,5-dimethylphenylacetic acid by converting p-xylene with chloroacetyl chloride into 2-chloro-1-(2,5-dimethylphenyl)ethanone, which is reacted with the compound of the formula (II) to give the compound of the formula (III), which is then rearranged to give a mixture of the compounds (IV) and (V), which is then hydrolyzed to 2,5-dimethylphenylacetic acid.