[en] The spatial and temporal evolution of the ³H levels in the water of the River Tagus in its passage through various regions of Spain and Portugal was analysed. Using mathematical time-series techniques, analytical expressions were obtained for the temporal trend and the periodicity present in the data.These expressions were used to determine the transit times between the sampling sites. The results indicated that the mean speed of ³H displacement was 12 km/month. The residence times of tritium in the water were also obtained. They were found to depend on the sampling points, with values ranging between 31 and 77 months. The concentrations of ³H varied cyclically at the six sampling points, with periods of around 24 months upstream of the Almaraz Nuclear Power Plant and about 12 months downstream