Time evolution of cross-linking clusters prepared by the sol−gel method was investigated by dynamic light scattering (DLS) and atomic force microscopy (AFM). The samples were prepared by the sol−gel synthesis of 1,8-bis(triethoxysilyl)octane (Tes-Oct) in the presence of phosphotungstic acid (PWA) as a super strong acid. The reaction batches were quenched at various reaction times, dtpmp scale inhibitor followed by dilution with a large amount of the solvent. DLS and AFM techniques simultaneously revealed that there exists a maximum cluster size, Rh and rAFM, respectively, for a specific PWA concentration. The information on the clusters density, ρcluster = (rAFM/Rh)3, generated during the reaction process was obtained by systematically comparing the DLS data obtained in the swollen state to the AFM data measured in the shrunken state of the cross-linked clusters. It was found that not only the cluster size but also ρcluster exhibited a maximum for [PWA]/[Tes-Oct] = 0.10 near the gel point of the Tes-Oct clusters. Since PWA is not only a super strong acid as a catalyst for the sol−gel synthesis but is also a proton conductor, the resulting materials can be utilized as a fuel-cell membrane. The ac impedance measurements revealed that the proton conductivity also exhibited a similar maximum at this specific PWA concentration, revealing the optimum condition for design of conducting membrane.