The precipitation process of the martensitic PH stainless steel Nanoflex during aging at 475 degrees C has been investigated with atom probe tomography. The composition of the matrix and the nanosized precipitates has been determined after aging for 5 min, 4 h, 40 h and 100 h. Also, the number density of the various precipitated phases has been measured. It is shown that the precipitation process is rather complex and that different precipitate phases form in direct contact with each other. At the early stages two families of Ni rich precipitates, Ni-3(Ti, Al) and Ni-3(Ti, Al, Si), form on Cu rich precipitates. Later, Cr rich precipitates form on the Ni rich precipitates. Eventually the Ni-3(Ti, Al, Si) precipitates dissolve and are replaced by Ni16Si7Ti6. Precipitates rich in Mo, most probably the quasicrystalline R' phase, are also formed in direct contact with other precipitates. The identity of the precipitates is discussed further in the paper. Even though the number density of the precipitates decreases during aging, the hardness does not decrease. This can probably be explained by an increased total volume fraction of precipitates as additional phases appear during aging.