The crystal structure of the compound bis [2,4-diamino-5-(p-chlorophenyl)-6-ethyl pyrimidinium hydrogen sulfate] monohydrate was studied by X-ray diffraction methods. The compound crystallises in orthorhombic system, space group P212121, a=18.531(2) A, b=16.190(2) A, c=10.777(2) A, V=3233.3(8), Z=4. The asymmetric unit shows the presence of two crystallographically independent pyrimethamine molecules, two hydrogen sulfate anions, and a water molecule. The hydrogen sulfate anions form a cyclic hydrogen bonded motif R2 2(8) with the 2-amino pyrimidine of the respective protonated pyrimethamine cations, through N–H···O hydrogen bonds. Thus the sulfate anions mimic the role of carboxylate anions observed in many aminopyrimidine-carboxylate interactions. This motif self assembles through DDAA array of quadruple hydrogen bonds, N–H···Cl, O–H···O and C–H···O hydrogen bonds.