AbstractAn investigation including the use of transmission electron microscopy has been carried out into the solid state transformations in Ti–Al–Ru alloys in the composition range 17–37 at.-%Al and 1–5 at.-%Ru. Ruthenium is shown to be a strongβstabiliser: a 5 at.-%Ru addition produced complete or substantially complete retention of metastableβon quenching from 1250°C (within the stableβrange). Three types of martensite can result by quenching from theβfield, depending on the composition: a hexagonal martensiteα′inα+α2alloys with 1 at.-%Ru, an ordered hexagonal (DO19) in alloys close to Ti3Al stoichiometry with 1 at.-%Ru; an orthorhombic martensite inα+α2alloys with 2 at.-%Ru. On heating martensite or retainedβin the range 1100–770°C, various transformations occur depending on composition and temperature, e.g. tempering of martensite toα2+βorα2+γ+ G, or reversion of martensite toβfollowed by precipitation of one or more of the phasesα,α2andγ.MST/1029