Copolymer of 3,4-ethylenedioxythiophene (EDOT) and diclofenac (DCF) was synthesized by chemical oxidation using FeCl3 in neutral medium. The copolymer exhibited good solubility in many organic solvents. The UV-vis spectrum of the copolymer revealed the presence of benzenoid and polaron or bipolaron state. Random distribution of the monomers in the copolymer was proposed based on the reactivity ratios of the monomers determined using Fineman-Ross and Kelen-Tudos methods. The rate of the copolymerization depended on the first power of each monomer. The FTIR spectrum showed the presence of hydrogen bonded N-H, quinone type structure of thiophene ring and C-Cl bond in the copolymer. Linear variation of anodic peak current at 560 mV in the cyclic voltammogram of copolymer thin film with scan rate indicated the adherence of electroactive copolymer. When the applied potential increased from -600 to 1000 mV, the color of the copolymer changed from neutral yellow to brown and to violet medium showing multicolor electrochromic behaviour. XRD results revealed crystallite copolymer with size 87 nm. SEM analysis confirmed uniform crystalline nature of the copolymer. The TGA, DTA and DSC studies suggested good thermal stability of the copolymer. (c) 2007 Wiley Periodicals, Inc.