Hydrogen diffusion in two low carbon steels with ferrite-pearlite or purely pearlitic mi-crostructures was studied with hydrogen permeation method. Strong differences in hydrogen permeation transients were observed. A much higher time lag for hydrogen diffusion is obtained for the purely pearlitic steel, which could not be explained with conventional hydrogen diffusion/trapping models. A new model, combining hydrogen diffusion/trapping and geometrical tortuosity of microstructure proved much better adequacy with experimental results. 1 Introduction It is well known that interactions between hydrogen and steel are sensitive to the mi-crostructure. Many studies relate the impact of microstructure phases (bainite, ferrite, pearlite, austenite or martensite) on the hydrogen diffusion and trapping in steel, as shown by recent publications [1, 2] on the subject. Major differences are observed for steels with different crystal structures. Austenitic steels, with face centred cubic (FCC) structure; always exhibit much higher hydrogen solubility and lower diffusivity [3] than body centered cubic (BCC) steels. Differences between BCC grades (ferrite, bainite, martensite) are usually less pronounced, and associated with different densities of trapping sites [4] (ferrite-cementite interfaces, dislocations…). In general, classical diffusion and trapping models [5-9] correctly describe the behaviour of these steels. However, in multi-phases alloys, very few works question the impact of morphology and connectivity of these phases on diffusion process. Recently, Osman Hoch et al. demonstrate the importance of grain-boundaries connectivity on the diffusion path of polycrystalline structures [10]. This recent work motivates investigation on hydrogen diffusion path of complex pearlitic microstructure. 2 Steels characterization In the present work, we studied two carbon steels used as flat wires. Their chemical composition is given in Table 1. The only difference between both steels is the carbon content which is 0.35 w% for steel A, and 0.7 w% for steel B. As a consequence, steel B presents higher mechanical properties with an ultimate tensile stress above 1000 MPa compared to 800 MPa for steel A.