As part of an ongoing series of experimental charge-density investigations into the intra- and intermolecular interactions present in compounds which undergo solid-state [2 + 2] cycloaddition reactions, the charge-density analyses of trans-cinnamic acid and coumarin-3-carboxylic acid are reported. Thus, high-resolution single-crystal X-ray diffraction data were recorded at 100 K for trans-cinnamic acid (sin theta/lambda(max) = 1.03 A(-1)) and coumarin-3-carboxylic acid (sin theta/lambda(max) = 1.19 A(-1)). In addition to the anticipated O-H...O hydrogen bonds weak C-H...O interactions were identified in both structures along with very weak intermolecular interactions between pairs of molecules that undergo solid-state [2 + 2] cycloaddition reactions upon irradiation.