Abstract: Process for the preparation of a heat treatment atmosphere by a catalytic reaction in a catalytic reactor including an inlet and an outlet between a first gas mixture comprising oxygen and a second gas mixture comprising a hydrocarbon for the purpose of supplying said atmosphere to a user site comprising at least one user location, comprising the steps of:a) feeding said first and second gas mixture at a given flowrate into the inlet of said catalytic reactor;b) continuously measuring the pressure of the heat treatment atmosphere obtained at the reactor outlet;c) comparing the pressure measured during step b) with a setpoint pressure level P.sub.