Monophasic gel with stoichiometric 3Al(2)O(3)center dot 2SiO(2) composition and gels with 0.99, 1.96, and 2.91 mol% La2O3 added were sol-gel derived. The crystallization path, structure evolution, microstructure, and morphology of calcined premullite powders and sintered ceramic bodies have been investigated as a function of La2O3 content and sintering temperature. In addition to mullite, spinel phase at about 980 degrees C, and a-alumina at above 1000 degrees C were determined; however, neither La2O3 nor La-related compounds had crystallized. The La2O3 predominately incorporated into the glassy phase, enhanced with La2O3 level, which affected both mullite structure and composition, as confirmed by electron microscopy, Rietveld structure refinement, determination of unit cell parameters, electron microscopy, and achieved density of the sintered bodies. Increased thermal treatment changes the alumina/silica ratio in mullite (towards 3: 2 below 1200 degrees C, and toward 2: 1 above), and decreases the mullite/amorphous ratio. Sintered dense ceramic bodies revealed a positive densification effect and increased sinterability as a result of the lanthanum-induced increase in glassy phase.