In the last years new combustion technologies have been developed to produce energy in an efficient and environmental-friendly way. In this background a very promising process is the Mild Combustion. It is characterized by highly diluted and pre-heated reactants. The use of high dilution degrees can lower the high reactivity of hydrogen and can allow for the use of this clean fuel. In particular way the use of steam as diluent covers a very important rule since this technology could be applied in steam turbines where hydrogen with oxygen can be added directly to the combustion chamber realizing an internal heating, hence a heat exchange with a high efficiency, and an environmental friendly process since the product of combustion is steam. The behavior of the hydrogen-oxygen system under mild condition and in presence of great amount of steam deserves a preliminary numerical study. Six selected hydrogen-oxygen combustion mechanisms have been computationally analyzed and compared among them on the basis of experimental ignition delay time data from shock- tube works available in literature. In a first analysis stoichiometric and lean hydrogen/air combustion has been considered as well as Mild Combustion conditions in presence of different amounts of steam. The temperature has been varied between 900 K and 1300 K and the pressure between 0.1 MPa and 0.9 MPa. Since deviations under nearly all analyzed conditions occurred an analysis of sensitivity has been carried through to characterize the most influential reactions under the various initial conditions. To improve the numerical results the reaction rate constants of the reaction HO2+H=H2O2+H2 have been varied as well as the third body efficiency of water in the breaking reaction H+O2+M=HO2+M. An improved result has been achieved.