Detailed X-ray crystallographic investigation of the reactive α- and the unreactive γ-polymorphs ofo-ethoxy cinnamic acid has been carried out along with that of the photodimer, α-truxillic acid. The molecule is quite planar in the α-form, but in the γ-form, the side groups deviate significantly from the plane of the benzene ring. The carboxylic groups form normal cyclic hydrogen bonds in the α-form and near-symmetric hydrogen bonds in the γ-form. The infrared spectrum of the α-form shows the characteristic features of the cyclic dimer, but that of the γ-form is entirely different, marked by the absence of the O-H stretching band in the 3000 cm-1 region. Charge density analysis throws some light on the structure and reactivity of the molecule in the two forms. The near-symmetric hydrogen bond in the γ-form is ionic and appears to restrict conjugation by way of distorting the molecule. This unusual feature keeps the cinnamoyl double bonds away from each other, rendering it photochemically unreactive. In the α-form, however, the double bonds have a closer approach. The cyclobutyl ring of the photodimer consists of weak single bonds, with the new pair being slightly longer.