JET experiments using deuterium and tritium have made it possible to study sawtooth stability in plasmas approaching thermonuclear conditions. Record fusion yields were obtained in discharges where the sawtooth was delayed. In a sequence of discharges designed to study alpha heating, a new phenomenon was observed: the sawtooth period increases with tritium concentration. The internal kink stability for both high performance and alpha heating experiment plasmas was studied. Calculations for the record fusion discharge at the time of maximum fusion power ( Pfus = 16 MW) showed that alpha particles make a significant stabilizing contribution to the potential energy of the m = 1 internal kink instability. The scaling of sawtooth period with tritium concentration implies a dependence on mean ion mass: possible reasons for such a dependence are considered. Calculations of the kinetic fast particle contribution to the kink potential energy indicate that the rise in sawtooth period with tritium concentration is likely to have arisen from two effects: an increase in the slowing down time of the beam ions, which in these plasmas is proportional to the mean ion mass, and an increase in the proportion of beam ions at the full injection energy.