Similar to expanded austenite, expanded martensite could be formed in the surface region of martensitic stainless steel after energetic nitrogen ion implantation at elevated temperatures up to 400°C, whereas a higher temperature of 420°C leads to CrN precipitates in the steel matrix. The influence of the process conditions and the alloy composition on the lattice expansion is investigated. A variable, anisotropic lattice expansion normal to the surface of up to 3.5% was observed, generally larger for (110) planes than for (200) planes. The additional formation of a large number of point defects for some process conditions can be inferred from the missing correlation between the relative X-ray intensity and the layer thickness.