Abstract: The extraction of acetic acid from an aqueous solution into solutions of quaternary amine,Aliquat 336,in different diluent,n-octanol,chloroform and kerosene,was investigated under various pH,concentration of Aliquat 336 and solute.The style of complex formation was discussed,and a model describing the equilibrium was proposed.The distribution coefficient,D,fitted the simple additive property of Aliquat 336 and the diluent.Diluent affected the D apparently; the D in chloroform and kerosene increased quicker than that in n-octanol with the Aliquat 336 concentration in organic phase.There was a maximum value for D against pH,which depended on the Aliquat 336 concentration only,and the pH value responding to the maximum D was 4.0～4.3 and 3.7～4.2 for 25% Aliquat and 50% Aliquat system,respectively.Acetic acid was bonded to Aliquat 336 by both ion exchange and chemical association,and diluent affected their parameters obviously.The apparent constant of ion exchange,K ₁,increased in diluent,especially in chloroform.The apparent constant of association,K ₂,increased in chloroform also,decreased in n-octanol,and kerosene affected K ₂ not apparently.