Surface-modified solids bearing groups of the formula I

SiR¹₂—R—NH₂  (I),

are prepared by reacting a solid having OH groups on a surface thereof with a cyclic silazane of the general formula II ##STR1##

where

R is a divalent optionally substituted hydrocarbon radical optionally containing heteroatom containing groups, and

R¹ is a hydrogen atom, or a monovalent optionally substituted hydrocarbon or hydrocarbonoxy group optionally containing heteroatom-containing groups. The surface-modified solids are preferably metal oxide particles. Such surface-modified particles are useful in numerous products.