There are provided curable resin sols having a curable polycyanate ester resin and at least 30 weight percent of an essentially volatile-free, colloidal dispersion of substantially spherical surface-modified silica nanoparticles; wherein the curable resin sol comprises no greater than 200 parts per million by weight alkali metal and alkaline earth metal ions based on the total weight of the silica nanoparticles and curable polycyanate ester resin. There are also provided compositions comprising such curable resin sol and reinforcing fibers, a process for preparing such compositions, and various articles made using such curable resin sols and compositions.