Procedures are given for chemically bonding an alkyl or aryl substituent to the surface of siliceous materials via a Grignard reaction. The silicon-carbon bond thus formed is thermally and solvolytically stable. Porasil C was halogenated and reacted with naphthyl magnesium bromide repeatedly to produce polynaphthyl-Porasil containing as much as 24% by weight of organic material. The benzyl-and naphthyl-Porasils undergo Freidel-Crafts acylation. Both cation and anion exchangers have been prepared from these substituted Porasils.