The best configuration for the D-T phase of operation in JET is assessed by analyzing two of the high-performance H-mode plasmas obtained in 1989. Several different simulations are performed using the same power levels as in the original D-D case: (a) 140 kV D beams are injected into a tritium plasma, (b) 160 160 kV T beams are injected into a deuterium plasma, and (c) a mixture of beam species is injected into a plasma initially 50:50 D:T. A second set of simulations involves increasing the beam power levels to their predicted design values while retaining the same thermal conductivities as previously. The effect of using two different particle transport models to prescribe the evolution of the thermal density profiles is investigated for each scenario.