Monitoring the nitriding treatment by analyzing directly the components’ surface state during the nitriding treatment is particularly interesting, since it allows a process monitoring and control based on the actual nitriding result. In the present study, two measuring methods are developed and combined with the aim of a direct surface state analysis during the nitriding treatment: the in situ X-ray diffraction (XRD) method and the photothermal radiometry. In order to validate the combined application of both methods during a nitriding treatment under controlled atmosphere, an experimental setup including a miniature nitriding furnace was developed. Two alloyed steels AISI 4140 and AISI H13 are treated with varying process atmosphere and nitriding potential leading to varying phase composition in the surface layer. As a result, the photothermal radiometry is shown to be sensitive with respect to the changing surface properties due to the growing compound layers and when porous layers are generated. It has a high potential to serve as surface sensor in industrial processes.