A transfer of nitrogen of about 20 at % is detected in very thin molybdenum films of about 200 nm thick coated on Si (100) wafers heated at 673 K and exposed to ternary (Ar-N2- H2) plasma. The nitrogen diffusion goes with a noticeable decrease of the remaining oxide layers in the whole film thickness. On the contrary, pure N2 gas exposure leads to a slight diffusion of nitrogen into the first molybdenum layers up to a depth of about 40 nm, only and an enhancement of oxygen amount also. Hydrogen species contained in the plasma reduce the oxide layers which act as nitrogen diffusion barrier. The nitrogen diffusion decrease with increasing distance of the workpiece surface from the centre of the discharge as well as the occurrence of nitrogen diffusion in molybdenum layers at room temperature highlight the role of NHx<3 active species of the plasma on the reactivity of the surface. The morphology of the nitrided surface consists of slightly smaller grains compared with those corresponding to untreated molybdenum films ranging in size from 30 to 50 nm wide. In contrast to substrates heated at 873 K, the tetragonal Mo2N structure has not been detected in molybdenum films heated at 673 K.