A variety of octadecylsilylated silica gel phases are prepared under various conditions, and their chromatographic properties are studied in order to carry out chromatographic characterization of commercial C18 packing materials. The test scheme, utilizing several sets of solutes, provides information about hydrophobicity, steric selectivity, and the extent of hydrogen bonding and electrostatic interactions. The results permit the estimation of how the commercial packing materials were prepared.