A polymorphic form of 5-[4-[2-(N-methyl-N-(2-pyridyl)amino)ethoxy]benzyl]thiazolidine-2,4-dione- , maleic acid salt (the "Polymorph") characterised in that it provides: (i) an infra red spectrum containing peaks at 1752, 1546, 1154, 621, and 602 cm.sup.-1; and/or (ii) a Raman spectrum containing peaks at 1751, 1243 and 602 cm.sup.-1; and/or (iii) a solid-state nuclear magnetic resonance spectrum containing peaks at 111.9, 114.8, 119.6, 129.2, 134.0, 138.0, 144.7, 153.2, 157.1, 170.7, 172.0, and 175.0 ppm; and/or (iv) an X-ray powder diffraction (XRPD) pattern which gives calculated lattice spacings of 6.46, 5.39, 4.83, 4.68, 3.71, 3.63, 3.58, and 3.48 Angstroms; a process for preparing such a compound, a pharmaceutical composition containing such a compound and the use of such a compound in medicine.