The isotope dependence of the H-mode power threshold and the energy confinement time have been examined in dedicated experiments on the Joint European Torus (JET) in hydrogen, deuterium and tritium plasmas. The results show that: 1) The H-mode power threshold is inversely proportional to the effective isotopic mass, M, of the plasma. 2) The edge electron pedestal temperature at the L to H-mode transition scales as M-0.5. 3) The global thermal energy confinement time, τth, does not depend strongly on M, ie. in the ohmic, L-mode, ELMy and ELM-free H-mode confinement regimes τth ∝ Mα with |α| ≤ 0.2, if the engineering parameters such as current, magnetic field, density, power and geometry are kept fixed. 4) In the ELMy H-mode regime the transport in the core and the edge of the plasma scale differently with M, ie. the core transport increases weakly with M whereas the edge transport decreases strongly with M.