Cocrystals constructed with p-coumaric acid (a phytochemical and nutraceutical compound) are investigated with xanthine compounds, caffeine and theophylline. Four cocrystals of p-coumaric acid with caffeine (1 : 1 and 1 : 2 stoichiometric ratios) and theophylline (two 1 : 1 polymorphs, Form I and Form II) were generated and their structures determined by single-crystal X-ray crystallography. The two theophylline cocrystals display synthon polymorphism, where both structures possess a carboxylic acid–imidazole heteromeric synthon; however, one polymorph also has a hydroxyl–carbonyl synthon (Form I), while in the other a hydroxyl–imidazole synthon (Form II) is present. Furthermore, the solid-state stability of the two p-coumaric acid : theophylline polymorphs was explored experimentally and computationally.