The microstructure and mechanical properties of hot rolled Fe-40 at-%Al based intermetallic alloys, with 0.1 at-%Zr and different additions of B (varying from 0.01 to 0.1 at-%), are characterised. The additions of Zr and B improve tensile properties at room and elevated temperatures. Increasing B content is also associated with a number of other effects. First, the fracture mode changes from intergranular decohesion to cleavage, which correlates with significant increases in the fracture toughness. Second, there is a certain stabilisation of dislocations arranged in parallel systems of slip bands, as shown by transmission electron microscopy. Numerous complex stacking faults on {100} planes are also observed in the alloy with the highest B content. Third, B is found to modify the formation of second phase particles; such particles (coarse and fine) are analysed by energy dispersive X-ray spectroscopy and electron energy loss spectrometry to obtain compositional information.