Abstract Direct in-situ observation of phase generation and growth during heat treatment cycles gives information independent e.g. of effects resulting from cooling and atmospheric changes of properties. In this investigation time resolved in-situ X-ray diffraction (XRD) analysis of growing nitride layers during plasma nitriding was conducted to gain experimental data of growing compound layers for different plasma nitriding parameters. With two gas mixtures of 5% N 2–95% H 2 and 25% N 2–75% H 2. plasma nitriding of an AISI 1045 steel was performed in the temperature range of 450 °C < T < 560 °C. The in-situ XRD-observation consisted of series of 50 to 60 single runs of phase analysis during a 3-h plasma nitriding treatment. Nitriding with the formation of nitride phases starts at different times, depending on the nitriding temperature and the gas composition in the plasma for the given plasma parameters pressure, voltage and current density. The higher the nitriding temperature and the higher the nitrogen content in the process gas the shorter is the time for the first detection of the γ′-Fe 4N-phase. Single phase γ′nitride layers were detected for the 5% N 2–95% H 2 gas mixture in a temperature range 450 °C < T < 560 °C. For the highest temperatures 540 °C and 560 °C and the gas mixture 25% N 2–75% H 2 the ε-Fe 2–3N phase occurred later in the plasma nitriding process. Assuming that nitride layers in plasma nitriding also grow by nucleation of small γ′ particles up to a complete layer, the experimental data fitted in a reasonable way in plots calculated for the incubation time of the γ′-phase during gas nitriding.