A study of mixed hydrogen-deuterium H-mode plasmas has been carried out in JET-ILW to strengthen the physics basis for extrapolations to JET D-T operation and to support the development of strategies for isotope ratio control in future experiments. Variations of input power, gas fuelling and isotopic mixture were performed in H-mode plasmas of the same magnetic field, plasma current and divertor configuration. The analysis of the energy confinement as a function of isotope mixture reveals that the biggest change is seen in plasmas with small fractions of H or D, in particular when including pure isotope plasmas. To interpret the results correctly, the dependence of the power threshold for access to type-I ELMing Hmodes on the isotope mixture must be taken into account.