An electrochemical permeation technique was performed on mild steel and S45C medium carbon steel. Mild steel shows higher permeation rate and diffusivity than medium carbon steel. The highest permeability and diffusivity for heat treated medium carbon steels are observed in the spheroidized condition and least permeability and diffusivity are in the martensitic condition. Annealed and normalized structures yield intermediate values of permeation rate and diffusivity. A monolayer of AgBr emulsion was coated on the determination site of a specimen to observe hydrogen transport in carbon steel. This hydrogen microprint technique also confirms that the main diffusion path is lattice and carbide/ferrite interfaces in pearlitic and spheroidized steels, and lath interfaces in martensitic steel.