A turbine of an organic Ranking cycle ORC is described, the turbine comprising a shaft supported by bearings and a plurality of seals arranged round the shaft for confining the operating fluid expanding in the turbine. The seals define at least four axially consecutive chambers. The operating fluid, with function of barrier fluid, is fed into one of the chambers adjacent to turbine stages; a gas, preferably inert, is fed into one of the chambers adjacent to the bearings, and the corresponding seals are gas seals. This configuration prevents the operating fluid from any kind of contamination by the lubricant used for the bearings, and avoids polluting the environment.