Thin cuprous oxide electrochromic films on the transparent conductive electrodes were prepared by chemical electroless method. The films cycled in K+-based electrolyte revealed typical red-ox peaks with higher intensity compared to those in the Li+ and the Na+-based electrolytes. The durability of the cuprous oxide due to cycling into LiClO4 was about 60 cycles. The thermal treatment of the films invoked decrease in red-ox peak intensity, and thus deterioration in the electrochromic properties. The response time of the coloration and bleaching to an abrupt voltage change from -4.5 to +4.5V and reverse was found to be in the range of about 10 s. The maximum light intensity modulation ability of the films, as the AM1.5 spectrum is taken for an input, was calculated to be about 65%. (c) 2007 Elsevier B.V. All rights reserved.