A variable transmission electrochromic architectural window includes spaced optically transparent elements confining an electrochromic medium including a solvent and a redox chemical pair in solution such that the redox chemical pair colors under the application of voltage and bleaches in the absence of the applied voltage. An ultraviolet stabilizer is included in solution for enhanced ultraviolet stability. The window has an electrochromically active large area of length dimension at least about 14 cm and a width dimension at least about 14 cm while exhibiting a steady state leakage current of less than about 2.68 amperes per square meter when a potential of one volt is applied such that coloration uniformity of the electrochromic architectural window is enhanced. Preferably, the electrochromic medium is based on the use of solvents comprising at least about 25% 3-hydroxypropionitrile, 3,3'-oxydipropionitrile, 2-acetylbutyrolactone, 2-methylglutaronitrile, 3-methylsulfolane and mixtures thereof.