Depending on the nature of the 4f element, six different lanthanide oxalate families were hydrothermally synthesized in the presence of hydrazinium ions. Four of them correspond to the general formula N2H5[Ln(C2O4)2]·nH2O but have different structural formulas according to the number of coordinated water molecules or hydrazinium ions and the structural arrangement, N2H5[La(C2O4)2] (1); N2H5[{Ln2(N2H5)}(C2O4)4]·4H2O, Ln = Ce, Pr, Nd, and Sm (2); N2H5[{Ln(H2O)}(C2O4)2], Ln = Sm, Eu, Gd, Tb, Dy, and Ho (3); N2H5[Ln(C2O4)2]·nH2O, Ln = Yb, n = 3, and Lu, n = 2 (5). The two others do not contain hydrazinium ions. Compound 4, obtained only with Ln = Er and Tm, contains a neutral lanthanide oxalate arrangement, [{Ln(H2O)}2(C2O4)3]. Finally, in the experimental conditions, crystals of compound 6 were obtained only for Lu, [{Lu(H2O)2}2(C2O4)3]·2H2O. For Ln = La to Ho, with coordination number CN = 9, 3D oxalate-lanthanide anionic frameworks are formed for the largest Ln, from La to Sm, and 2D networks are obtained for the smaller, from Sm to Ho. For Ln = Er to Lu, with CN = 8, 3D oxalate-lanthanide frameworks are formed; a 2D network is obtained only for the smaller lanthanide, Lu. The structures of compounds 1, 3 for Ln = Tb (3-Tb) and Ho (3-Ho), 4 for Ln = Er (4-Er), 5 for Ln = Yb (5-Yb) and Lu (5-Lu), and (6) were determined from single-crystal X-ray diffraction data in space groups P21/c, Pbca, P21/n, Fddd and P1̅, respectively. Thermal behaviors were studied by thermogravimetric analysis and high temperature powder X-ray diffraction. Optical properties were measured by UV-vis and IR spectroscopy.