SummaryThe conditions for in situ chemical modification of commercially available thin-layer plates have been varied and the influence thereof upon reversed phase plate properties has been studied. The reaction variables studied were: average silica pore size, silica water content, use of tri-or monochlorosilanes as modifiers, reaction temperature, and additionof base.Coverage and activity of residual silanol groups was studied on modified plates.Desired plate properties are high speed of development in aqueous solvents and low content of active residual silanol groups. Under the conditions tested, the lowest amount of residual silanols was achieved at the expense of the speed of development in aqueous media and vice versa.