The effects of Fe-doping on the modulated structures in Bi 2.5Sr 1.5CuO y (2.5/1.5-phase), as well as Bi 2.2Sr 1.8CuO y (2.2 /1.8-phase), are examined by electron diffraction and high-resolution electron microscopy. The Fe-free Bi 2.5Sr 1.5CuO y as well as the specimens doped with Fe up to 20% of Cu in the initial ratio, crystallizes in an orthorhombic superlattice with modulation periodicity b s =4.2 b. Deviation from the orthorhombic symmetry accompanied by a shortening of the modulation periodicity is observed in the range of Fe from 20% to 40%. Specimens with Fe from 40% to 50% crystallize in a monoclinic commensurate superlattice with b s =4.0 b. The Fe-free Bi 2.2Sr 1.8CuO y , on the other hand, has a monoclinic superlattice with b s =4.8 b. Partial substitution of Cu with Fe results in a gradual decrease of the modulation periodicity down to b s =4.2 b for 50% substitution. At the same time, transition of the symmetry is induced by Fe substitutions and an orthorhombic superstructure is observed with Fe from 30% to 50%.