Pptd. Mg silicates, a com. silica, and emulsion-pptd. silica (to be dry-modified), were modified with 3-10 wt. parts of 3-thiocyanatopropyltrimethoxysilane (I), N-phenyl-3-aminopropyltrimethoxysilane (II), methacryloxypropyttrimethoxysilane (III) or vinyltrimethoxysilane (IV). Products were examd. by FTIR spectra. The Mg silicate treated with 10 wt. parts I and II lost entirely the original silanol groups and acquired C-H from the modifiers. The com. silica modified with 10 wt. parts silane had considerably less silanol groups and much of n-octyltriethoxysilane groups. The emulsion-pptd. silica modified with I and IV had silanol groups much suppressed and C-H due to modifiers. Covalent bonds could be assumed to have formed between the silica or silicate surface and the modifier.