The properties of JET plasmas obtained under similar L mode conditions, but using different ion species, are compared. In hydrogenic limiter discharges with NBI heating, particle confinement and sawtooth activity display a clear isotopic dependence. However, the gain in global energy confinement from hydrogen to deuterium is less pronounced than that implied by popular L mode scaling relationships and the improvement appears to be only partly due to a reduction in the local conductive heat flux. No significant change in energy or particle confinement is observed when deuterium is replaced by 3He