The microstructure of an Fe-40 at.% Al-0.5 at.% B-0.7 at.% C alloy in the as-extruded condition has been examined; in addition to the matrix B2 phase, four other phases were found to be present. These were Fe AlC with the 3 0.5 perovskite structure, Fe B, a tetragonal boride with the CuAl structure, 2 2 graphite (free carbon), and a new tetragonal phase that is probably coherent with the matrix. An orientation relationship exists between this tetragonal [ ] [ ] ) ) phase and the B2 matrix of the form 100 // 100 and (001 //(001 . From p m p m selected-area diffraction patterns, it is noted that this new tetragonal phase has lattice constants that can be related to the matrix lattice parameter a as a 4 a 0 p 0 and c a . Stacking-fault-like features are also present in the as-extruded p 0 material and these lie on the cube planes of the matrix. The new tetragonal phase precipitates frequently on the partial dislocations bounding these 'faults'; the precipitates themselves are rod shaped with a square cross-section. The axis of the rod coincides with the 100 directions in the matrix while the sides of the square cross-section lie along the matrix 110 directions. After annealing the extruded material, this new phase is no longer observed, leading to the conclusion that this phase is metastable.