A series of arylphenylcyclopropenones has been synthesised, and the relative proportions of the isomeric acids obtained by base-catalysed cleavage determined. With the meta- and para-monosubstituted compounds this ratio can be correlated with the σ-constants of the substituents. ortho-Substituents cause preferential cleavage to the 2-phenyl-3-arylacrylic acid irrespective of the electronic properties of the substituent.