The effects of dissolved organic matter (DOM), water soluble organic matter derived from sewage sludge, on the sorption of atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-trazine) by soils were studied using a batch equilibrium technique. Six paddy soils, chosen so as to have different organic carbon contents, were experimented in this investigation. Atrazine sorption isotherms on soils were described by the linear equation, and the distribution coefficients without DOM (Kd) or with DOM (Kd*) were obtained. Generally, the values of Kd*/Kd initially increased and decreased thereafter with increasing DOM concentrations of 0-60 mg DOC x L(-1) in soil-solution system form. Critical concentrations of DOM (DOM(np)) were obtained where the value of Kd* was equal to Kd. The presence of DOM with concentrations lower than DOM(np) promoted atrazine sorption on soils (Kd* > Kd), whereas the presence of DOM with concentrations higher than DOM(np) tended to inhibit atrazine sorption (Kd* < Kd). Interestingly, DOM(np) for tested soils was negatively correlated to the soil organic carbon content, and the maximum of Kd*/Kd (i.e. Kmax) correlated positively with the maximum of DOM sorption on soil (Xmax). Further investigations showed that the presence of hydrophobic fraction of DOM evidently promoted the atrazine sorption on soils, whereas the presence of hydrophilic DOM fraction obviously tended to inhibit the atrazine sorption. Interactions of soil surfaces with DOM and its fractions were suggested to be the major processes determining atrazine sorption on soils. The results of this work provide a reference to the agricultural use of organic amendment such as sewage sludge for improving the availability of atrazine in soils.