Argillites are one of the rock types studied by French authorities for their confining properties for the isolation of radioactive wastes. One of the main objectives of such study is the better understanding of water transport through rocks with very low water content and hydraulic conductivity, using modeling of tracer profiles. This article presents the protocol developed and applied for acquiring data on chloride in interstitial water of the Toarcian argillites in Tournemire (southern France). This protocol is based on laboratory experiments involving diffusion process and on modeling. Experimental data obtained during transient and steady parts of diffusion allow for the assessment of the diffusion coefficient and initial concentration in pore water, respectively. Profiles for both have been acquired along the geological sequence; they are used in part 2 of this article for proposing a hydrogeological model where diffusion appears to be the main process for mass transport through the argillites and for comparing deuterium and chloride transport.