The photoreactivity of a series of crystalline p-phenelendiacrylic acid monoesters has been investigated. The ethyl derivative forms photostable crystals whereas cycloaddition reactions yield a mixture of a p-cyclophane and oligomeric products from n-decyl crystals. Crystal structure analysis for both compounds shows that the packing is driven by the steric disparity between the alkyl chains and the p-phenelendiacrylic acid. However only the more planar n-decyl molecules are correctly aligned for the photocycloaddition reaction.