This work deals with the analysis of the formation of porous spherical silica particles without adding any surfactants. The use of a wide range of measurement techniques allows the particle formation process to be tracked with regard to the particle size, structure and density. The analysis of the pore inlet diameter distribution based on the Kelvin effect indicates a change in the particle structure at higher reaction temperatures. Measurements with UV-Vis spectroscopy and TEM showed a collapse toward smaller particle sizes within the first minutes of the reaction. The higher the synthesis temperature, the more pronounced the collapse is and the sooner it takes place. Time-resolved SAXS data correlate a drastic increase in scattering intensity with an increase in particle density due to a collapse in particle size. (C) 2018 Elsevier B.V. All rights reserved.