AbstractStudies of minor phase evolution in a 12CrMoVNb martensitic steel, creep tested for durations of up to 100000 h at 550°C and 600°C, have revealed the presence of a hitherto unknown complex nitride precipitate having a tetragonal unit cell similar to that of Z phase, previously only reported in certain niobium bearing austenitic steels. This paper describes the electron diffraction and X-ray microanalysis studies leading to the discovery of this phase, and the conclusion that it is a new form of Z phase enriched in vanadium. A modified crystallographic model is proposed for this new complex nitride phase, as are the precipitation sequences leading to its formation.MST/3575