Positron beam analysis (PBA) was performed on cold-rolled Fe0.94Ni0.04Ti0.02 foils, which were subjected to different thermal treatments in an atmosphere of a gas mixture of NH3+H-2 (nitriding). The nitriding of the samples in the alpha -region (alphaN) of Lehrer diagram for the Fe-N system produced a large decrease of the central part of the Doppler broadened annihilation gamma -peak (S-parameter) and an increase of the contribution to the wings of the peak (W-parameter). The effect was much more pronounced for the (x-nitrided samples than for samples annealed in vacuum at the same temperature. A reduction of the alphaN-samples by annealing in H-2-atmosphere brings the S-parameter to a higher value. Further nitriding of alphaN-samples in the gamma'-region (alphaN+gamma 'N) of the Lehrer diagram increases S and lowers the W-parameter compared with the alphaN-samples. The changes in S- and W-parameters are interpreted on the basis of the evolution of microstructure of the films during the processing.