In our laboratory various electrochromic windows (ECWs) have been investigated using mainly tungsten oxide (WO,), polyaniline (PANI) and prussian blue (PB) as electrochromic materials in combination with poly(2-acrylamido-2-methyl-propane-sulphonic acid) (PAMPS) as a solid proton-conducting electrolyte. The ECWs have been characterized by AC-impedance, linear sweep voltammetry and spectroelectrochemical studies in the 290-3300 nm spectral region. The ECWs have the following general multilayered structure: Glass/ITO/EC1/IC/ EC2/ITO/Glass, where ITO = indium oxide doped with tin, IC = ionic conductor, EC1 is either PANI or PANI including PB, and EC2 is WO3. The best of these ECWs has been able to regulate up to 56% (typical 50%) of the transmission of the total solar energy in the 290-3300 nm spectral range. The combination of the two electrochromic materials PANI and PB has been shown to be mutually beneficial in such a way that the colouration of the window is enhanced by the addition of a layer of PB onto PANI, while the adhesion of PB is improved by the presence of PANI. The energy consumption of the ECW is about 0.01 Wh/m(2) for one complete cycle (-1.8 V/1.2 V). The switching time for 90% colouring/bleaching is typically 10-30 s. A PANI/PB//WO3 window has been operated for about 50 days ( similar to 3700 complete cycles) without substantial loss of transmission regulation, though with an increase in switching time (10 min.). Spectra from individual layers in the ECWs have been recorded by making holes in one or two of the electrochromic layers. In this way (the hole method), it has been possible to study the transmission regulation properties for each electrochromic material separately in complete solid state windows. In addition, spectra for complete windows have been simulated by adding contributions from individual electrochromic layers.