Herein, carbon distribution and grain-boundary diffusion processes are studied in a variety of FeCr alloys and steels with different chromium content and microstructure, based on magnetic after-effect measurements performed in a broad temperature range, from 100 to 1000 K. The existence of three metastable carbon positions in the lattice is revealed in the FeC alloys. Carbon as interstitial in the lattice, segregated at the dislocations and grain boundaries, is represented by the relaxation peaks at about 269, 432, and 607 K, respectively. The relaxation process due to the grain-boundary self-diffusion is clearly distinguished from those mentioned earlier, and is observed to be at about 643 and 681 K in low and high carbon Fe, respectively. Addition of Cr has a twofold effect: a) for Cr concentrations higher than about 3 wt carbon-related relaxation processes completely disappear from the spectra, most probably due to formation of carbides, b) the solute grain-boundary diffusion relaxation peak appears in the spectra at about 800 K, with an activation energy that is directly dependent on the Cr content. Activation energy of the solute grain-boundary diffusion is found to be generally smaller in ferrite–martensite microstructure in comparison with fully ferrite alloys.