Abstract: This paper deals with the crystal structures of Gadolinium, Terbium, Erbium and Thulium acetylacetonate trihydrate M-L[Gd(C₅H₇O₂)₃·3H₂O, Tb(C₅H₇O₂)₃·3H₂O, Er(C₅H₇O₂)₃·3H₂O,Tm(C₅H₇O₂)₃·3H₂O] which have been determined by X-ray diffraction method. All of them are isomorphous, monoclinic, p2₁/n, space group and 2 = 4, with dimensions Gd-L, a=11.235(4), b = 21.934 (9) , c = 8.337(2) Å, β=100.24(2)°; Tb-L,a = 11.228( 3), b = 21.882 (9) , c = 8.342 (2) Å, β=100.36(2)°; Er-L, a = 11.223(2), b = 21.798 (6) , c = 8.343 (2) Å, β=100.57(2)°; Tm-L, a=ll.210(4), b = 21.758 (9) , c = 8.357(3) Å, β = 100.67( 3 )°. The structures were solved by Patterson and Fourier techniques and refined by block least-squares to conventional factors R(Gd-L) =0.0988, R(Tb-L) =0.0851, R(Er-L) = 0.0589 and R(Tm-L) =0.0838(R = ∑|F₀-F_c|∑F₀).The results of the determinations of the crystal structures showthat the rare earth metal ion in the four complex molecules is eight-coordinates, being bounded to three acetylacetonate rings and two water molecules.Third uncoordinated water links a coordinated water by hydrogen bond. This uncoordinated water also links to another uncoordinated water of another complex molecule by hydrogen bond, related by a center of symmetry. The molecules are thus connected in pairs. The coordination polyhedron about the rare earth metal ion is a distorted square antiprism.