The ortho-, para- and meta-chloro-substituted N-chlorophenyl-2-phthalimidoethanesulfonamide derivatives, C16H13ClN2O4S, have been structurally characterized by single-crystal X-ray crystallography. N-(2-Chlorophenyl)-2-phthalimidoethanesulfonamide, (I), has orthorhombic (P2(1)2(1)2(1)) symmetry, N-(4-chlorophenyl)-2-phthalimidoethanesulfonamide, (II), has triclinic (P (1) over bar) symmetry and N-(3-chlorophenyl)-2-phthalimidoethanesulfonamide, (III), has monoclinic (P2(1)/c) symmetry. The molecules of (I)-(III) are regioisomers which have crystallized in different space groups as a result of the differing intra-and intermolecular hydrogen-bond interactions which are present in each structure. Compounds (I) and (II) are stabilized by N-H center dot center dot center dot O and C-H center dot center dot center dot O hydrogen bonds, while (III) is stabilized by N-H center dot center dot center dot O, C-H center dot center dot center dot O and C-H center dot center dot center dot Cl hydrogen-bond interactions. The structure of (II) also displays pi-pi stacking interactions between the isoindole and benzene rings. All three structures are of interest with respect to their biological activities and have been studied as part of a programme to develop anticonvulsant drugs for the treatment of epilepsy.