The oxidation behaviour of P91 steel with and without Al-CVD-FBR coating was investigated by means of mass spectrometry and TG measurements. All samples were oxidized at 650 °C for 190 h in an atmosphere of Ar + 40%H 2O. TG-MS experiments were conducted in a closed steam loop in order to obtain information about the oxyhydroxides formation as reaction between coatings and steam. During the oxidation of Al-CVD-FBR/P91 sample were detected volatile species as, FeO(g), Al 2O 2(g) around 25 h of test. AlOOH(g) and FeOOH(g) species around 100 h. FeOOH(g), Fe(OH) 3(g) species around 125 h. FeOOH(g), Fe 2O 3H 2O(g) species around 150 h. CrO 2(g), volatile species around 175 h. Finally, Al(OH) 2(g), Al(OH) 3(g), Fe(OH) 3(g) volatile species around 190 h. From those results the role of the coating element could be established and optimized for the coating durability. The morphology/composition and structure of oxidized samples were also studied using the SEM-back scattered electron images/EDS and XRD techniques. © 2010 Springer Science+Business Media, LLC.