We have investigated the crystal structures of several 1-naphthylmethylammonium salts of unsaturated carboxylic acids, i.e., sorbate, crotonate, tiglate, and methacrylate as the diene and vinyl monomers. In the crystals, the monomer molecules stack in a similar fashion to make a lamellar structure consisting of alternating naphthylmethylammonium cation and carboxylate anion layers. For the construction of the lamellar structure, weak intermolecular interaction such as pi-pi stacking and CH-pi interaction is important as well as the formation of robust two-dimensional hydrogen bond network. We discuss the relationship between the molecular packing in the crystals and photopolymerization reactivity of the unsaturated carboxylic acid derivatives in the solid state.