The diffusion of tritiated water (HTO) in Opalinus clay (OPA) samples from bore cores from the Benken area ( Northern Switzerland) was studied using the radial through- and out-diffusion technique. Results from inverse modelling of out-diffusion data for HTO indicated the presence of two preferential diffusion pathways: a fast and a slow one. Analysing through- diffusion data, however, provides hardly any information concerning a second transport-relevant porosity. Only by also analysing the out-diffusion phase can finer details of the diffusion process and information on sample heterogeneity be recognised. The extracted values for the effective diffusion coefficient are in the order of 3 x 10(-11) m(2) s(-1) for the faster transport porosity and roughly an order of magnitude smaller for the slower type of porosity. We had to account for tritium sorption on the clay minerals by a small but non-zero K-d-value in the order of 10(-5) m(3) kg(-1) in order to reproduce the data with acceptable precision. In the model applied both porosities are considered as being independent from each other. Roughly 30% of the tracer diffused through the second, slower porosity; such a fact might be interesting for future performance assessments for radioactive waste repositories hosted by clay formations. Based on our present picture from water-saturated OPA, on a microscopic scale three different kind of waters can be discriminated: free water, double layer water and interlayer water. However, using HTO as tracer only, it could not be deduced which type of water-filled pores finally account for the transport-relevant porosity.