Natural Na bentonite contg. 65% montmorillonite and synthetic Na montmorillonite were mixed 2 hrs. in aq. 50% EtOH with 2 wt. parts of 3-chloropropyl (I) or 3-methacryloxypropyl trimethoxysilane (II). The av. particle size of modified montmorillonite was 10-15 lm vs. the 10 in the original. The II-modifted bentonite had particles up to 60 lm. The interlayer distance in Na montmorillonite was changed by I only slightly, 1.244 vs. 1.265 nm, and likewise in bentonite, regardless of the coupler type. Up to 1000 degrees C, wt. loss was 1%. In I- or II-moditled montmorillonite, the loss was 2.8 and 4.7% and the DTG peaks at 390-460 and 345-465 degrees C, resp., were attributed to the decompg. silanes. Only the silane surface was affected and penetration of silane mols. into the interlayer space was insignificant.