Two metal dusting experiments were carried out at 570 °C on 800HT and HR120 alloys, for more than 6000 h. The tests were designed to run at different total pressures and gas velocities but similar carbon activities and oxygen partial pressures. For a given alloy, shorter average incubation times and larger mass losses were observed at high pressure. For both tests, HR120 alloy underwent greater mass losses and exhibited a higher pit density. For nearly all samples, pit densities greatly differed between both sides of the specimens. Therefore, the carbon and oxygen activities alone are not sufficient to evaluate the aggressiveness of a metal dusting environment. Greater degradation was the result of the association of a higher gas velocity with a higher total pressure and a finer alloy grain size.