In this work we present the dehydration study of the complexes K3Fe(C2O4)3·3H2O and K3Cr(C2O4)3·3H2O. Thermal analysis was carried out using TGA and DSC techniques, and was found that the dehydration occurs in two steps. X-ray powder diffraction analysis indicated that the trihydrate phase crystallize in the monoclinic space group P21/c with unit cell parameters a = 7.762(1) Å, b = 19.937(3) Å, c = 10.353(1) Å, β = 107.75(1)°, while the anhydrous phase crystallize in the monoclinic space group P21/m (Nº 11) or P21 (Nº 4), with unit cell parameters a = 14.510(5) Å, b = 13.669(7) Å, c = 5.936(2) Å and β = 105.86(8)°.