Microstructural stability, microhardness and oxidation behaviour of an in situ reinforced Ti-8.5Al-1B-1Si (wt %) alloy was examined in both air and argon environments. When exposed for up to 5760 min at temperatures below the alpha/alpha + alpha(2) transius, hardening occurred in both air and argon environments. The increase in hardness in the air heat-treated samples is attributed to a combination of solid-solution strengthening due to the oxygen and the precipitation of the alpha(2) phase, while the increase in hardness in the argon heat-treated samples is primarily due to the precipitation of the alpha(2) phase. On the other hand, when heat treated above the alpha/alpha + alpha(2) transius, after an initial increase in hardness there is a drop in hardness which is attributed due to elimination of the alpha(2) phase and a decreased contribution of boron and silicon in the matrix towards the solid-solution strengthening by virtue of coarsening of the TiSi2 and TiB reinforcements. Oxidation of the alloys follows a parabolic oxidation law when oxidized both in an environment of flowing air and static air with the primary oxidation product being TiO2. The activation energy for oxidation is 200 kJ mol-(-1) in an environment of flowing air and 303 kJ mol(-1) in static air. The difference in activation energy arises from the difference in the availability of oxygen at the reaction front in the two cases.