The solid phases in the theophylline-oxalic acid co-crystal system have been investigated by thermal analysis techniques, X-ray diffraction analysis (XRD) and solubility measurements. The work includes the 2 : 1 co-crystal of theophylline and oxalic acid, polymorphs of theophylline, the theophylline monohydrate and solid oxalic acid. The DSC curve of the co-crystal presents two endothermic peaks, one at about 230 degrees C where the oxalic acid in the co-crystal decomposes and carbon dioxide is liberated, and another one at 279 degrees C where the remaining theophylline melts. At equilibrium with the solid co-crystal, the theophylline concentration is only 60% of the corresponding value for the pure solid theophylline. Using the solubility data, the standard Gibbs free energy of formation of the co-crystal from the pure solid phases is estimated to be approximately -6.0 kJ mole(-1). In a chloroform/methanol mixture (4 : 1 v : v) the commercial form of pure theophylline (Form II) has been observed to transform into a more stable non-solvated form (Form I). The data suggest that the polymorphs are enantiotropically related with an approximate transition temperature of 70 degrees C. The 2 : 1 theophylline-oxalic acid co-crystal can be successfully produced by cooling crystallisation in chloroform/methanol mixture (4 : 1 v : v) and by slurry conversion crystallisation also in more benign solvents.