The thermal stability of two-phase (alpha + gamma) lamellar microstructure in Ti-Al-Mo PST (polysynthetically twinned that has single colony) crystals, containing C or Si, was investigated. In addition, the variation of a-phase volume fraction in Ti-Al-Mo-(C,Si) systems was investigated at several temperatures. Ti-46Al-1.5Mo-0.2C and Ti-46Al-1.5Mo-1.0Si alloys did not recrystallized ('stable' in this paper) during heat treatments at various heating rates and temperatures. Moreover, the a-phase volume fractions of Ti-46Al-1.5Mo-0.2C and Ti-46Al-1.5Mo-1.0Si alloys which were stable compositions, changed less than those of Ti-47Al and Ti-46Al-1.5Mo alloys which were unstable compositions. The instability of the latter alloys was caused by their relatively higher variation of a-phase volume fraction during heating. Therefore, it is suggested that the variation of a-phase volume fraction is an important factor in controlling the thermal stability of lamellar microstructure. (C) 2004 Kluwer Academic Publishers.