The　title　compound　cytosinium　isophthalate　(C-H2IA)　self-assembly　of　cytosine　(C)　and　isophthalic　acid　(H2IA)　in　aqueous　media　has　been　synthesized　and　the　crystal　structure　with　a　reasonable　protonation　state　is　redetermined.　Single-crystal　X-ray　diffraction　analysis　reveals　that　each　asymmetric　unit　contains　one　protoned　cytosine　molecule　and　one　deprotoned　isophthalic　acid.　The　proton　transferred　from　carboxylic　acid　to　the　pyrimidine　ring　is　disordered　across　an　inversion　center　with　occupancy　of　0.5　and　the　proton　located　to　one　of　the　carboxylate　group　lies　on　an　inversion　center　shared　by　two　crystallographically　equivalent　oxygen　atoms.　In　addition,　the　cytosine　molecules　are　connected　by　complementary　hydrogen　bonds　to　form　a　one-dimensional　tape　structure.　The　neighboring　isophthalic　acids　are　connected　via　hydrogen　bonds　between　carboxyl　groups　to　form　a　one-dimensional　lattice　like　tape.　Furthermore,　the　adjacent　organic　base　tapes　and　organic　acid　tapes　are　stacked　one　with　another　through　π-π　stacking　interactions　to　form　a　three-dimensional　supramolecular　structure.　Interestingly,　C-H2IA　displays　a　green　phosphorescence　in　solid　state　at　room　temperature　with　the　lifetime　of　0.7　s　determined　by　time　resolved　studies,　indicating　that　supramolecular　C-H2IA　is　a　potential　pure　organic　phosphorescent　luminogens.　©　2020　Fujian　Institute　of　Research　of　the　Structure　of　Matter.　All　rights　reserved.