Five transition metal derivatives of maleic acid with general formula, M2+(C4H3O4−)2⋅4H2O (M2+=Mn, Fe, Co, Ni, and Zn) were prepared by slow evaporation of the aqueous solution at room temperature. Their X-ray powder diffraction patterns were recorded and evaluated. These materials are isostructural and crystallize in a triclinic unit cell. The volume of the cells vary linearly between that of the Ni complex [V=314.65(7) Å3: a=5.1769(8) Å, b=7.317(1) Å, c=9.140(2) Å, α=108.42(2)°, β=104.61(1)°, γ=92.87(1)°] and the volume of the Mn-derivative [V=330.30(8) Å3: a=5.322(1) Å, b=7.375(1) Å, c=9.752(2) Å, α=115.48(2)°, β=106.64(2)°, γ=86.63(2)°].