The consequences of ageing at 400 and 475 A degrees C on alpha' precipitation and corrosion resistance of a novel Cr-Mo ferritic SS with high Mo content were investigated. Combination of X-ray diffraction and Mossbauer spectroscopy results indicates the appearance of a small amount of a paramagnetic phase as a result of the ageing. This phase is iso-structural with the alpha phase and is thus associated with a Cr-rich alpha' phase. The appearance of this phase is related to a decrease of the resistance to corrosion as studied by DL-EPR because the presence of this paramagnetic phase leads to discontinuities in the oxide layer thus reducing corrosion protection and exposing the steel to the action of the corrosive solution.