The　effect　of　the　zeta　(zeta)　potential　of　activated　carbon　on　the　adsorption　of　cationic　dyes　from　aqueous　solution　has　been　studied.　The　pH(zpc)　of　the　zero-point　charge　of　activated　carbon　has　been　measured　by　microelectrophoresis.　Plots　of　saturated　amount　adsorbed　for　methyl　green　and　methyl　violet　on　activated　carbon　versus　zeta　potential　have　also　been　determined.　The　results　show　that　the　electrostatic　interaction　between　cationic　dyes　and　the　surface　of　activated　carbon　has　a　great　effect　on　adsorption　capacity.　When　the　zeta　potential　of　activated　carbon　is　lower　than　-30　mV,　the　electrostatic　attraction　between　cationic　dyes　and　the　carbon　surface　greatly　increases　the　adsorption　capacity.　However,　when　the　potential　of　activated　carbon　is　between　-30　and　+60　mV　the　electrostatic　force　between　dyes　and　the　carbon　surface　is　diminished　and　the　adsorption　capacity　of　activated　carbon　is　approximately　unchanged.　The　capacity　is　significantly　reduced　due　to　the　electrostatic　repulsion　between　cationic　dyes　and　the　carbon　surface　when　the　zeta　potential　is　higher　than　+60　mV.　The　adsorption　kinetic　and　thermodynamic　parameters　as　functions　of　pH　have　been　determined.　The　mechanism　for　adsorption　of　cationic　dyes　on　activated　carbon　has　also　been　studied.　(C)　1994　Academic　Press.　Inc.