In this work the influence of aluminium on the nitriding behaviour of High Speed Steels (HSS) was investigated. The steels were plasma-nitrided in an atmosphere with a hydrogen to nitrogen ratio of 3 to 1. The thickness and the formation of the nitriding layer were examined by means of optical light microscopy as well as scanning electron microscopy. These examinations were supported by hardness measurements. In order to characterize precipitates present in the nitriding layer, atom probe tomography was used. It was found that the thickness of the diffusion layer and the ‘white layer’ decreases with increasing aluminium content. This is due to the increasing amount of nitride forming elements. The more nitrides are formed the less nitrogen is available for the diffusion into the bulk. The hardness measurements showed that the hardness in the nitriding layer increases and that the nitriding case depth decreases with rising aluminium content. This is caused by the precipitation of nano-scale nitrides containing aluminium. However, the rising aluminium content leads to a reduced size of the precipitating nitrides.