The properties of JET plasmas obtained in similar conditions, but using different ion species, are compared. In hydrogenic limiter discharges with NBI heating, particle confinement and sawtooth activity display a strong isotopic dependence. However, the improvement in global energy confinement from hydrogen to deuterium is only approximately 20%, not consistent with the square-root dependence tE - Ai1/12 often found in L-mode scaling relationships. No significant change in energy or particle confinement is observed when deuterium is replaced by helium-3.