Maraging steels belong to the group of ultra-high strength steels and are often used in critical aerospace, automotive and tooling components. By applying a surface treatment such as nitriding, the fatigue and wear resistance can be improved. The precipitation reactions in Fe-Ni-Co-Mo maraging steels are studied after different heat treatments. The formation of precipitates during simultaneous nitriding and aging is compared with the precipitations formed during aging prior to nitriding. Data from electron backscatter diffraction, x-ray diffraction, transmission electron microscopy and atom probe tomography were used to characterize the microstructure of the layers. An effective way to visualize the nitrided layer is by the band slope maps derived from the electron back scatter diffraction pattern. The distortion in the diffraction pattern is associated to the volume misfit between the matrix and the nitrides, which also leads to generation of compressive residual stresses in the nitrided layer. Both nitrides and intermetallic precipitates were detected and studied. These results contribute for a better understanding of the nitride formation process in maraging steel in different processing conditions.