While p-ethynylbenzoic acid (EBA) has been suggested to be reactive in the solid state, the structure-reactivity relationships and the nature of the product(s) have remained unknown. Crystallized from toluene, EBA is a monoclinic crystal, space group P2(1)/n, a = 3.8684(4), b = 6.2329(4), c = 30.190(3) angstrom; beta = 90.281(8)degrees; V = 727.90 angstrom(3). The crystal structure contains linear chains and short intermolecular contacts between acetylene moieties, which provide a structural basis for initiation of topochemical reactivity. Heating of EBA leads to an amorphous poly(phenylacetylene) (PPA) derivative. The absorption and emission spectral characteristics of the polymer are discussed. Analogous to PPA, a subsequent thermal process leads to a cyclic trimer. Differential scanning calorimetry of EBA exhibits at least four exothermic processes, and these are discussed in terms of the polymerization and subsequent processes that ultimately lead to the cyclotrimer.