The adsorption isotherm of polyethyleneimine(PEI) modified diatomite was studied using UV spectrophotometry．The surface of diatomite was modified with polyethyleneimine by using impregnation method

and the trapping behavior of the modified diatomite for phend was investigated by using 4-aminoantipyrine method．The experiment results showed that negatively charged diatomite particles had very strong adsorption effect for cationic macromolecules of PEI

and it was attributed to the strong electrostatic interaction．The adsorption isotherm fits the Freundlich equation．After modification

the electric properties On the surface of diatomite particles changed factually

and the isoelectrie point moved from pH=2．0 to pH=10．5．In acidic solutions

phenol exists as molecules

and the modified particles adsorb phenol throush hydrogen bonding interaction．However the hydrogen bonding interaction between nitrogen atoms on PEI chains and phenol is weak because of the high degree of protonation of polyethyleneimine macromolecules

so the adsorption quantity is low．In basic solutions

phenol exists as phenoxy anions

and the modified particles adsorb phenol through electrostatic interaction．However the electrostatic interaction between PEI and phenoxy anions is very weak owing to the low degree of protonation of polyethyleneimine macromoleeules

so the adsorption quantity is much lower．The modified particles exhibit very strong trapping effect for phenal through the cooperation of strong electrostatic interaction with hydrogen bond effect in neutral solutions

and the saturated adsorption capacity can be as high as 92 mg/g．