In the title compounds, 2-amino-4-(2-chloro-4,5-di­methoxy­phenyl)-1,3-thia­zole, C₁₁H₁₁ClN₂O₂S, (I), and 4-(2-chloro-4,5-di­methoxy­phenyl)-2-methyl-1,3-thia­zole, C₁₂H₁₂ClNO₂S, (II), the dihedral angles between the thia­zole moiety and the chloro­aryl group are 51.61 (10) and 8.44 (14)°, respectively. This difference is a consequence of intermolecular hydrogen bonds forcing the stabilization of a twisted rotamer in (I). Substitution of the amino function by a methyl group precludes these contacts, giving a flat rotamer in (II).