A highly water-soluble hyperbranched polymer with viologen units (HB-1) was synthesized for application to electrochromic (EC) display. Electrochemical and spectroscopic properties of HB-1 were evaluated by absorption spectra and cyclic voltammetry. They were compared with those of a low molecular weight viologen molecule (1). We fabricated a simple EC window by a thin (300-500 nm) solid film of HB-1 and an electrolyte solution to demonstrate the EC behavior. By successively applying opposite bias voltages to it, reversible color changes between colorless and purple were observed with good contrast, whereas blue color was observed for 1 upon reduction. The colored state in HB-1 was ascribed to the generation of radical cation dimer species of viologen units due to high local concentrations in HB-1. When the bias was turned off after achieving the colored state, no fading was observed in the EC window. This result indicates that the radical cation dimer species of HB-1 in solid films were stable against disproportionation or reaction with oxygen even though no active protection was made in this EC window. Since the present EC device has such a memory effect, bias voltage is needed in writing and erasing processes alone, which is very useful in practical applications. (C) 2010 Elsevier B.V. All rights reserved.