AbstractIn order to increase the thermal efficiency of advanced power generation systems higher operation temperature and pressures should be used. The P91 steel has excellent mechanical properties from room temperature up to approximately 600°C due to a distribution of a fine-sized Nb and V carbonitrides. In this context, special attention must be put on the welds. The failure of a component may be initiated in a localized and microscopic region of the weldments, either during welding or in service. This is due to the differences in the behaviour of the various regions of the weld because of the heterogeneity in its microstructure.In the present work the characterization of precipitates in the fusion zone, heat affected zone and base material was performed on a circumferential weldments by means of transmission electron microscopy. The weldments were obtained using a semiautomatic welding process under gas mixture protection with two different filler materials.