The paper deals with the processes of dissolution of highly disperse iridium powders with sizes from 6 nm under hydrothermal conditions in hydrochloric acid solutions. Iridium powders with surface areas from 1 to 46 m² / g were synthesized in closed systems by reduction of aqueous solutions potassium hexa­chloro­iridate(III). At temperatures from 170°C to 195°C, the dissolution processes of iridium powders in solutions of hydrochloric acid with additives of oxidants were investigated. The data regarding to the degree of dissolution of iridium powders as a function of dispersion, temperature, time, oxidizer are obtained.