Adsorption of Cr V I and p-methoxyphenol (PMP) on soil colloids at different pH media was studied. The resulting k 1 and n of 1.89x10 2 and 0.53 (r 2 =0.99) and k 2 and b of 0.13 and 1.25x10 3 (r 2 =0.96) were obtained from Freundlich (Q=k 1 C a q n ) and Langmuir [Q=k 2 bC a q /(1+k 2 C a q )] simulation equations, respectively, for Cr V I adsorption on soil colloids (pH 4.20). The adsorption of PMP on soil colloids in pH 5.72 media was simulated by five different equations and the results indicated that the Fritz-Schluender one (r 2 =1.00) was the most suitable among them. Adsorption quantity of Cr V I and PMP on colloids increased with increasing acidity in the pH range of 3.5-9.0. Study of Cr V I adsorption kinetics indicated that the adsorption equilibrium of Cr V I was reached rapidly within 2 h. In pure aqueous solution, Cr V I reduction by PMP was observed only when the media's pH was lower than 4.0. Oxidation and reduction reaction between Cr V I and p-methoxyphenol obviously occurred when soil colloids were involved in this system, even at pH>=7.0, which strongly suggested that minerals in soil colloids acted as catalysts to speed the reaction of Cr V I and PMP. The oxidized product of PMP by Cr V I , extracted by chloroform in acid media and analyzed by gas chromatography-mass spectrometry, was identified as benzoquinone. The reaction included two steps of one electron process.