Six steel grades, T22, T23 (2 1/4 Cr), T91, T92, T122 (9-12%Cr) and TP347 (austenitic 18%Cr) were exposed during 32 weeks (= 5376 h) in pure water vapour at temperatures of 500, 550, 600 and 650 degreesC. The corrosion kinetics, determined by gravimetric measurements (by oxides descaling), follows a parabolic law, except for the austenitic steel and the 2 1/4 Cr steels at the highest temperatures, The corrosion rates of the six steels may be high : 0,1 mm/year for the 9-12 %Cr at 650 degreesC for example. The favourable effect of chromium is strongly less high in steam than in other media (air, combustion gases...). The stainless steel TP347 in water vapour has a better corrosion resistance but after all mediocre because of an important scaling phenomenon.