The kinetics of the primary phase formation in a glass ceramic base glass near the cordierite composition in the MgO–Al2O3–SiO2 system, with additions of ZrO2 and TiO2 as nucleating agents, have been investigated by combining different small-angle scattering techniques. Phase formation in the base glass heated isothermally at 1093 K has been studied in order to elucidate the structural processes that take place during controlled nucleation before volume crystallization begins. Complementary results of X-ray diffraction, transmission electron microscopy and electron microprobe analysis reveal that in the final stages of annealing, different crystalline phases, which are dispersed in the residual glassy phase, are developed. The largest particles have sizes of the order of several hundreds of nanometers and consist of magnesium petalite as well as a high-quartz solid solution. Besides these crystals, two populations of smaller particles, the sizes of which can be resolved by small-angle scattering, were detected and found to be enriched with TiO2 and/or ZrO2. The growth of particles of intermediate size was studied in situ. The radius, R, of each of these particles increases with increasing treatment time, t, according to the power law Rt1/2, indicating diffusion-limited independent growth. The scattering patterns are modified by diffusion zones surrounding the growing particles. The characteristics of the diffusion zones are evaluated by fitting appropriate model scattering curves to the experimental scattering data. The structural arrangement of the Ti and Zr ions was studied by contrast variation with the help of anomalous small-angle X-ray scattering and small-angle neutron scattering. As revealed by the contrast variation experiments, Ti ions are concentrated in the interface between the intermediate-sized particles and the glass matrix, whereas Zr is homogeneously distributed in these particles. Additionally, the Zr ions participate in the formation of the smallest particles, which are crystallites enriched with ZrO2. Finally, the catalyzing effect of TiO2 and ZrO2 on the nucleation and volume crystallization of the glass ceramic is discussed.