The microstructures of the L12 titanium trialuminides at low temperatures were studied using a number of single crystals with various Al-Ti-Fe compositions, all of which lie in the nominal single phase L12 Field at 1200 °C. Five different second phases were found to be in equilibrium with the L12 matrix, namely , (Al,Ti)3Fe, Al3Ti, Al2FeTi, Ti2NAl and Al2Ti+Fe. Small volume fractions of the first two phases are often seen in compounds containing relatively low Ti contents. The Al2FeTi, a so-called T phase, was observed at relatively high Fe contents. The Ti2NAl does not seems to be sensitive to the Al-Ti-Fe composition, it exists to some extent in all the alloys used in this study. Ti2NAl and the interface with the L12 matrix are found to be brittle and provide the sites for crack initiation. The Al2Ti+Fe phase has been observed in many compounds containing high Ti contents (>25 at.%), and has a large hardening effect. Like binary Al2Ti, the phase possesses a tetragonal structure of the Ga2Hf-type, and forms plates on the cube planes of the LI2 matrix. The crystallographic relation between the Al2Ti and the Ll2 matrix was determined to be (100)p//(100)m.and (010)p//(010)m. Porosity is also commonly seen in these alloys, and has a very destructive impact on ductility.