Carbon diffusion was studied in carbon-supersaturated surface layer of commercial ferrite-martensite Cr-Mo steel P91 and in two model ferrite materials: in pure α-Fe, and in ferrite Fe-Cr alloy. The carbon surface layer, the thickness of which was about 40 nm, was physical-vapor-deposited and isothermal diffusion anneals were carried out at selected temperatures between 573 K and 1073 K. The depth profiles of carbon were measured by SIMS. It was found that the carbon diffusion proceeded much more slowly in carbon-supersaturated surfaces than in matrices with equilibrium carbon concentration. The carbon diffusion coefficients obtained, D, were close to literature values reported for C diffusion in carbide phase. Measured values of D in pure α-Fe, α-Fe-Cr and P91 steel above 773 K were identical. However, significantly higher values of D were measured in P91 below the temperature of 773 K, which was ascribed to different mobility of substitution elements.