In this study we report the structure of supercritical H2O–SiO2 fluid composed of 50mol% H2O and 50mol% SiO2 at 3000K and 2400K, investigated by means of ab initio molecular dynamics of models comprising 192 and 96 atoms. The density is set constant to 1.88g/cm3, which yields a pressure of 4.3GPa at 3000K and 3.6GPa at 2400K. Throughout the trajectories, water molecules are formed and dissociated via the network modifying reaction 2 SiOH=SiOSi+H2O. The calculation of the reaction constant K=[OH-]2/[H2O][O2-] is carried out on the basis of the experimentally relevant Qn species notation and agrees well with an extrapolation of experimental data to 3000K. After quench from 3000K to 2400K, the degree of polymerization of the silicate network in the 192-atom models increases noticeably within several tens of picoseconds, accompanied by release of molecular H2O. An unexpected opposite trend is observed in smaller 96-atom models, due to a finite size effect, as several uncorrelated models of 192 and 96 atoms indicate. The temperature-dependent slowing down of the H2O–silica interaction dynamics is described on the basis of the bond autocorrelation function.