The crystal structures of p-chloro-trans-cinnamic acid (I) and β-(p-chlorophenyl) propionic acid (II) have been determined from X-ray diffractometer data. Both molecules are approximately planar and the crystal packing is similar when viewed down the b axis. Crystals of both compouns are monoclinic, space group P2₁/a, with Z= 4 in a unit cell of dimensions; for (I): a= 32·813(9), b= 3·890(1), c= 6·538(1)Å, β= 95·94(2)°: and for (II): a= 30·024(6), b= 5·071(1), c= 5·728(1)Å, β= 98·70(1)°. The structures were refined by full-matrix least-squares to R0·059 [(I), 1160 observed reflections] and 0·052 [(II), 1372 observed reflections]. The cinnamic acid crystallizes in the β-form with head-to-head packing so that the ethylenic bonds lie near each other, thus explaining why a photo-dimerization to a β-truxinic acid can occur. There is disorder of the carboxy hydrogen atom in (I) but it is localized in (II).