Trialuminide alloys of elements such as Ti. Nb or Zr are of particular interest as materials for high temperature usage because their density is very low and specific strength and elastic rnoduli are then very high. This report concentrates on recent work on Al3Ti alloys which have been alloyed with ternary elements such that the higher symmetry ordered cubic structure is obtained, leading to somewhat easier operation of deformation mechan isms and hence improved ductility and toughness.Fine details of the crystal structure of cubic trialuminides are considered here and it is shown that the materials generally possess some remnant tetragonal chemical ordering which can affect their me chanical behaviour. In addition the compositional range over which a stable single phase is retained is shown to be extremely small, such that in most cases the materials examined show some form of microstructural instability. These instabilities affect the mechanical behaviour of the materials, for exarnple producing general strengthening. leading to precipitation hardening du ring hig h temperature testing, and causing age hardening instabilities during high temperature static or dynamic testing.Such structural instabifity feads to significant modifications at superdislocations, affecting both the dislocation cores and their associated APB’s. Failure for these cubic materials still occurs at very small plastic strains and seems to be determined by difficulties of superdislocation creation near a propagating crack rather than by problems of suitable dislocation configuration and mobility. Possible ways to enhance ductility and toughness by alloying and microstructural modification will be discussed. Trialuminide alloys of elements such as Ti. Nb or Zr are of particular interest as materials for high temperature usage because their density is very low and specific strength and elastic rnoduli are then very high. This report concentrates on recent work on Al3Ti alloys which have been alloyed with ternary elements such that the higher symmetry ordered cubic structure is obtained, leading to somewhat easier operation of deformation mechanisms and therefore improved ductility and toughness. Fine details of the crystal structure of cubic trialuminides are considered here and it is shown that the materials generally possess some remnant tetragonal chemical ordering which can affect their me chanical behavior which show their form of microstructural instability. These instabilities affect the mechanical behavior of the materials, for exar nple producing general strengthening. leading to precipitation hardening du ring hig h temperature testing, and causing age hardening instabilities during high temperature static or dynamic testing.Such structural instabifity feads to significant modifications at superdislocations, affecting both the dislocation cores and their associated APB’s. Failure for these cubic materials still occurs at very small plastic and and seems to be determined by difficulties of superdislocation creation near a propagating crack rather than by problems of suitable dislocation configuration and mobility. Possible ways to enhance ductility and toughness by alloying and microstructural modification will be discussed.