Precursor material is hydrolyzed at a high temperature region of the reactor as a first step, and then the middle temperature oxidation of the reactant flow is performed in the combustion zone of the reactor in the second stage, in which the introduction of the combustion oxygen or air, unwanted an instantaneous cooling to prevent the formation of byproducts the resulting gas flow is carried out, a two-step process for the decomposition of the precursor material with a steam plasma in three areas reactor. Associated device non-transition dc steam plasma torch, an external cooled 3 regions steam plasma reactor means for introducing the precursor material into the plasma torch within the plasma plume, means for introducing combustion air or oxygen to the combustion region, including a means for cooling the reactant mixture is located in the means and the reactor of the discharge end for discharging the reactant mixture from the reactor.