Abstract The paper describes the reaction of arylacetones IX with triethyl phosphonoacetate, producing esters of 4-aryl-3-methyl-2-butenoic acid, X, and 4-aryl-3-methyl-3-butenoic acid, X'. Their hydrolysis gave a mixture of the isomeric acids I and I', whose composition was investigated by 1 H NMR spectra. Also prepared were 3-methyl-3-phenyl-2-propenoic acids, II, 2-aryl-2-hydroxypropanoic acids, IV, and substituted α-benzyloxyphenylacetic acids, V. ...