The porous and high surface area of lanthanum oxide was obtained as a final decomposition product of lanthanum acetylacetonate tetrahydrate. The decomposition course in dry nitrogen was thoroughly studied. Thermal processes occurring throughout the decomposition range (100-800 degrees C) were monitored by thermogravity; differential thermal analysis and infrared spectroscopy gaseous products. These processes were characterized on the basis of the solid products analyses, using X-ray diffractometry and IR-spectrometry. The results showed La(C5H7O2)(3) completely decomposed to La2O3 at 730 degrees C, through amorphous and unstable intermediates La(CH3COO)(C5H7O2)(2) at 190 degrees C, La(CH3COO)(2)(C5H7O2) at 225 degrees C, La(CH3COO)(3) at 285 degrees C, La-2(CO3)(3) at 390 degrees C and crystalline and stable La2O2(CO3) at 430 degrees C. Gas phase decomposition products included propyne, acetone, carbon oxides, methane and isobutene. Methane and isobutene resulted from interfacial reactions involving the initial product (acetone). The ultimate products, La2O3 at 700 degrees C and 800 degrees C, are crystalline porous solids and having a surface area of 21 and 45 m(2)/g respectively.