The effects of Mo additions on the microstructure and the room temperature deformation behavior of polysynthetically twinned (PST) crystals of TiAl were studied in order to get a basic conception for alloying additions on the two-phase TiAl compounds with the lamellar structure. It was found that the Mo additions in TiAl PST crystals increase both the yield stress and tensile elongation to fracture but the increase in yield stress deppend on the angle $Phi$at which the lamellar boundaries lie from the loading axis. The large difference in yield stress between specimens deformed parallel($Phi = 0^circ$)or perpendicular($Phi = 90^circ$) to the loading axis and those deformed in intermediate orientations could be plained by the difference in Mo content between the TiAl and the $$Ti_{3}Al$ phases. It was also found that the Mo-doped specimens with intermediate orientation fail by cracking zigzag across to the lamellar boundaris, which is the same fracture mode as that of binary specimens with intermediate orientations tested in vacuum This suggests that Mo atoms are thought to play a role to reduce the environmental embrittlement of binary PST crystals, resulting in increasing the tensile ductility.