At the catalytic tritiation of 5-Formyluracil (5-FU), different compounds are generated. The kind of the species depends on the reaction conditions. 5-FU is one of the derivatives of uracil which are slightly approachable to a catalytic hydrogen isotope exchange. This isotope exchange in the formyl group seems to be favoured above the 6-position in the uracil ring, because of its greater reactivity. After separation of the impurities, we obtained tritiated 5-FU with a specific radioactivity of nearly 1480 GBq/mmol and a radiochemical purity >98%. The pressure of tritium gas was <25 kPa. The catalyst we used was PdO(1.4%)/BaSO4. With the same catalyst we obtained 5-Hydroxymethyl[hydroxymethyl-3H]- uracil by raising the pressure of tritium gas to 50 kPa. Hydrogenation of the 5.6 double bond occurred by an additional raising of the pressure to 75 kPa and we obtained 5-Hydroxymethyldihydrouracil (5-DHMU) [1].