The tritium balance based on pressure measurements in the divertor was performed for the DTE1 campaign carried out at JET in 1997. This is a complementary analysis to the integrated tritium inventory already reported for a full day of experiments. The particle balance shows that the T flux retention is about 1.26×10 21 T s −1 during a plasma pulse (∼65% of the injected flux). Outgasing between pulses reduces this T inventory to about 30% of the injected fluence. This detailed gas balance also shows that the retention of hydrogen species depends on the plasma isotopic ratio. However, during the D clean-up pulses, the amount of T removed by isotopic exchange is shown to be limited to about 2×10 23 T. Finally, the total particle balance (D+T) is shown to be independent of the plasma isotopic ratio.