The paper addresses the problem of the effect of crack opening on the ability of chloride ions to diffuse along a crack. Experimental tests are carried out on an inert material (brick) to prevent crack self-healing. A mechanical expansive core is used to generate cracks of constant width across the thickness of the sample. Five cracked specimens with crack openings ranging from 21 to 128 μm were exposed to a chloride solution for 10 h and then removed to determine the total chloride concentration perpendicular to the crack path. The results show that crack opening significantly influences the ability of chloride ions to diffuse along a crack. Indeed, the chloride profiles indicate impediment of chloride-diffusion capacity in cracks less than 53 μm in width. It was found that this crack-opening threshold agrees with the critical crack opening measured in a uniaxial tensile test that characterizes the interaction phenomena between the fracture surfaces.