Results are presented from a series of dedicated experiments carried out on the Joint European Torus (JET) in tritium, deuterium-tritium, deuterium and hydrogen plasmas to determine the dependence of the H-mode power threshold on the plasma isotopic mass. The Pthres ∝ Aeff–1 scaling is established over the whole isotopic range. This result makes it possible for a fusion reactor with a 50:50 DT mixture to access the H-mode regime with about 20% less power than that needed in DD. Results on the first systematic measurements of the power necessary for the transition of the plasma to the Type I ELM regime, which occurs after the transition to H-mode, are also in agreement with the Aeff–1 scaling. For a subset of discharges measurements of Te and Ti at the top of the profile pedestal have been obtained, indicating a weak influence of the isotopic mass on the critical edge temperature thought to be necessary for the H-mode transition.