The scaling of the energy confinement in H-mode plasmas with different hydrogenic isotopes (H, D, D-T and T) is investigated in JET. For ELM-free H-modes the thermal energy confinement time τth is found to decrease weakly with the isotope mass (τth ~ M0.25 ± 0.22) whilst in ELMy H-modes the energy confinement time shows practically no mass dependence (τth ~ M0.03 ± 0.1 ). Detailed local transport analysis of the ELMy H-mode plasmas reveals that the confinement in the edge region increases strongly with the isotope mass whereas the confinement in the core region decreases with mass (τthcore ∝ M -0.16) in approximate agreement with theoretical models of the gyro-Bohm type (τgB ~ M-0.2).