The kinetics of the reaction, a-SiO2 + t-ZrO2 --> ZrSiO4, were monitored by isothermal HTXRD experiments. A series of diffractograms were recorded at constant temperature (1250 degreesC, 1300 degreesC, 1350 degreesC, 1400 degreesC), after heating the precursor powder. Integrated intensities of the peaks relating respectively to the ZrSiO4 (200) and tetragonal t-ZrO2 (101) planes, were used to obtain the conversion rate. According to the shape of the curves, lambda = f(t), the limiting step of the zircon formation is not the nucleation process, but, in fact, diffusion of silicon species through a zircon layer, from an external continuous or granular silica phase to the surface of zirconia grains. To account for the results obtained, a model of a contracting sphere is proposed: the system is described as either a series of spherical t-zirconia grains surrounded by a shell of zircon, which is covered by amorphous silica, or as spherical t-zirconia grains surrounded by a shell of zircon, which is in contact with spherical amorphous a-SiO2 particles. In the latter case, fast superficial diffusion of Si4+ and electrons originating from silica grains on the external surface of the zircon layer produce a homogenisation of the silicon concentration. The validity of this model is confirmed by the fact that the experimental data, lambda = f(t), fit the kinetic law of Valensi and Carter, considering chemical diffusion through the reaction product (oxide layer) as the rate limiting step. Consequently, the apparent diffusion coefficient of silicon can be estimated.