A hydrolytic side reaction in hydrated iridium chloride films heat-treated on titanium supports was discovered by simultaneous thermal analysis, emission FTIR and SIMS techniques. In the presence of water vapour the degree of hydrolysis increases significantly in the outermost part of the film leading to the formation of oxidic species at low temperatures. In a wet oxygen atmosphere encapsulated hydrogen chloride as a hydrolysis product reacts with the support to form titanium chloride species segregated in the surface region of the coating. In addition to the formation of hydrolysis products, the presence of surface species like iridium carbonyl and iridium carboxylate was also confirmed in the coating.