Electrochemical copolymerization of diphenylamine (DPA) with 2,5-dimethoxyaniline (DMA) was performed in 4 M H2SO4 aqueous solution for different feed ratios of DPA using cyclic voltammetry. The deposition rate of DPA-DMA copolymer is higher than that of homopolymers (PDPA and PDMA). (DMA-co-DPA) film was deposited using electrochemical polymerization as conducting film on indium tin oxide (ITO) electrode and used as an electrode in an electrochromic device. From the deconvoluted results of X-ray photoelectron spectroscopy, an azeotrope composition of DPA-DMA copolymers exists at DPA feed ratio of 0.5. Poly (3,4-ethylenedioxythiophene): poly (4-styrenesulfonate) (PEDOT:PSS) was spin-coated on ITO as the other electrode. Polyurethane ionomer (PUI) blended with LiClO4 was used as solid polymer electrolyte. A total solid electrochromic device was assembled as follows: ITO P(DMA-co-DPA) vertical bar vertical bar LiClO4-PUI vertical bar vertical bar PEDOT:PSS ITO. The columbic efficiency of the devices reached 84% for P(DMA-co-DPA) film with azeotrope composition. The optical contrast (Delta T,%) of the single electrode and the device were determined by UV-visible spectroelectrochemical studies. A value (31%) of Delta T in color upon switching the potential from -1.0 to + 1.0 V (vs PEDOT) was noticed for this device. The device was pale yellow at -1.0 V and blue at + 1.0 V. (c) 2006 The Electrochemical Society.