The structural evolution during a sol to gel transition reaction in the K2O-Al2O3-SiO2 system was investigated by using in situ small angle X-ray scattering (SAXS) technique. The results are interpreted as evidence of the presence of at least two different gel phases. The analysis shows that the primary phase is composed of small particles with characteristic size of similar to 3.0 nm, and considerable size and shape variation. The primary phase is identified as a random phase in the Debye sense. The primary particles aggregate into larger formations with spherical symmetry. The size of larger aggregates (second phase) increases continuously in the course of gelation and levels off after reaching similar to 15.0 nm. The volume fraction of the second phase keeps increasing until the end of the measurement which is far after the gelation (3-fold of the gelation time).