The base molecule of the title co-crystal, C7H10N2O2S·C7H6O3, is essentially planar, with a maximum deviation of 0.0806 (14) Å for all non-H atoms. The acid molecule is also nearly planar, with a dihedral angle of 8.12 (14)° between the benzene ring and the carboxy group. In the crystal, the acid molecules form an inversion dimer through a pair of O—H⋯O hydrogen bonds with an R 2 2(8) ring motif. The pyrimidine molecules are linked on both sides of the dimer into a heterotetramer via O—H⋯N and C—H⋯O hydrogen bonds with R 2 2(8) ring motifs. The heterotetramers are further linked by weak C—H⋯O hydrogen bonds, forming a tape structure along [1-10]
