Cyclic voltammetry is used to deposit films of hydrous iridium oxide onto glassy carbon electrodes from a basic solution of saturated iridium(III) oxide. An acidic solution of Ir(OH2)(2)Cl-4(-) is first prepared from either IrCl63- or IrCl62-. When this solution is made basic, iridium(IV) oxide is deposited by oxygen generated from oxidation of hydroxide. A strongly adherent, bright blue deposit of iridium oxide is formed after 10 to 20 scans between -0.3 and + 1.0 V vs. Ag/AgCl. The modified electrodes exhibit a super-Nernstian response to pH similar to that observed for iridium oxide grown electrochemically on iridium substrates. The pH response is reproducible and remains stable over a period of several days.