We concluded that the application of NaN3 as a biocide in sorption-desorption experiments has to be carefully evaluated. The magnitude and rate of the transformation of atrazine by NaN3 are of crucial importance, especially in experiments conducted for a long periods of time and with high NaN3 concentrations (>100 mg l−1). This issue is vital in sorption experiments in which the compound of interest is indirectly measured (e.g., monitoring the radioactivity of 14C-labeled analyte). In those cases, it is impossible to evaluate the observed transformation and the calculated sorption parameters can be incorrectly analyzed.