Samples of PM (powder metallurgy) plain iron were superficially enriched with Mo during a sintering process using a DC discharge. The Mo atoms from the cathode produced an enriched layer of approximately 15-20 mu m thick, and it was enriched with up to 2.0 at.% Mo. Subsequently, the samples were plasma nitrided in a gas mixture (N-2/H-2) at different temperatures and nitrogen concentrations. The effect of the molybdenum on the plasma nitrided layer of sintered iron was investigated. Abnormal nitride morphologies that developed in the surface layer were observed. The presence of Mo that was substitutionally dissolved in ferrite influences the nucleation and growth of the iron-nitride compound layer. The microstructure and (local) composition changes of the layers were investigated using scanning and transmission electron microscopy (SEM and TEM), X-ray diffraction (XRD), energy dispersive X-ray (EDX) and glow discharge optical emission spectroscopy (GDOES) analyses. To evaluate the mechanical properties, Vickers microhardness tests were conducted along the sample cross-sections. According to the nitriding conditions, submicroscopic fcc Mo2N-type nitrides that are coherent with the alpha-Fe matrix develop, as confirmed by the TEM analysis and by the broadening of the diffraction lines in the X-ray diffractogram. Molybdenum nitrides, gamma-Mo2N, with an fcc structure and sphere-like shapes were observed on the sample surface where the Mo concentrations were higher. (C) 2015 Elsevier B.V. All rights reserved.