The paper describes the studies developed by ENEL Produzione Ricerca about a diffusion flame gas turbine combustor fed with mixtures of natural gas and hydrogen. CFD studies using FLUENT carried out the changing fuel composition from natural gas to pure hydrogen for an extensive high pressure test campaign with a Gas Turbine in the ENEL Facility Sesta. While modifying the fuel mixture, the NOX emission level shows an evident increase while the metal temperature distribution changes negligibly. This work was financed by the fund established by the Italian Government to support research activities of general interest of the electric sector (Ministerial Decree 26 January 2000).