A series of new compounds LaxCo4Sb12 with the skutterudite crystal structure, where 0 < x < 1, was prepared using modulated elemental reactants. The evolution of the reaction was followed using calorimetry and X-ray diffraction. The modulated elemental reactants were found to inter-diffuse at temperatures below 150 degreesC forming an amorphous reaction intermediate. The crystallization temperature and nucleation energies for the transformation of the amorphous intermediate into the skutterudite crystal structure were both found to vary linearly with lanthanum content. The lattice parameter of the resulting skutterudite structures increased as the lanthanum filling was increased. The relative intensities of the Bragg reflections indicate that the lanthanum content in the skutterudite structure increases as the lanthanum content of the modulated elemental reactant increases.