A recent series of JET experiments using deuterium and tritium [1], has made it possible to study sawtooth stability in plasmas approaching thermonuclear conditions. It was found that large sawtooth crashes in ELM-free H-mode plasmas may seriously limit the achievable fusion power. Record fusion yields were obtained in discharges where the sawtooth was delayed. In addition, in a sequence of discharges designed to study a-heating [2], an interesting new phenomenon was observed: the sawtooth period increases with tritium concentration. We have studied internal kink stability for both high performance and a-heating experiment plasmas, and considered possible reasons for a dependence of the sawtooth period on the mean ion mass.