The paper deals with phosphoric acid organic esters such as commercially available diphenylphosphate and custom made cyclic phosphoric ester of oligo(propylene oxide). The N,N-dimethylformamide and propylene carbonate solutions of these compounds were tested for potential application as electrolytes in a protonic electrochromic cell with tungsten trioxide as an active compound. As a reference, a solution of phosphoric acid of identical concentration was used. Cyclic chronovoltamperometry studies were used to test both the stability window of the electrolytes on a platinum electrode and the stability of indium tin oxide and tungsten trioxide electrodes against the electrolyte solution. The proton intercalation process was also studied. The experiments were performed both for dry solutions (vacuum drying of compounds) and for ones, which were on purpose contaminated with water. Additionally, the dc conductivity of the studied systems was measured for the whole possible operational temperature range. Generally, it was observed that the ester solutions give a better photochromic response of the WO3 electrode but are, more aggressive to its surface. This observation is valid especially for the water-contaminated systems. (c) 2006 Elsevier B.V. All rights reserved.