Sodium azide (NaN(3)) is one of the biocides commonly used to inhibit microbial growth during sorption experiments. However, a few reports have suggested that NaN(3) can react with the analyte of interest. In this study, the interactions of NaN(3) with triazine herbicides were investigated and the effect of atrazine transformation on its sorption to soil was evaluated. The concentration of atrazine in the presence of NaN(3) decreased significantly over period of time. After 14 days, only 38% of the initial atrazine concentration (10 mg l(-1)) was detected in a solution containing 1,000 mg l(-1) NaN(3) at pH 5.5. The magnitude and the rate of atrazine transformation increased with increase in NaN(3) load and with decrease in pH. In contrast to atrazine behavior, the concentrations of prometon and ametryn did not change during the experiment. GC/MS analysis indicated that the chlorine atom of atrazine is replaced by the azide group yielding 2-azido-4-(ethylamino)-6-(isopropylamino)-s-triazine. Atrazine transformation by NaN(3) significantly affected sorption of herbicide to soil. The presence of NaN(3) affects indirectly the sorption of atrazine due to competitive effect of its derivative. Our results demonstrated that the application of NaN(3) as a biocide in sorption-desorption experiments must be carefully evaluated. This issue is vital for sorption experiments conducted over long periods of time or/and with concentration of NaN(3) higher than 100 mg l(-1).