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 NaN3 significantly affected sorption of herbicide to soil. The presence of NaN3 affects indirectly the sorption of atrazine due to competitive effect of its derivative. Our results demonstrated that the application of NaN3 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 NaN3 higher than 100 mg l−1.