Abstract: A method for measuring the concentration of isothiazolones in aqueous systems including removing sample interferences by lowering the pH of a sample collected from the aqueous system containing isothiazolones and filtering the sample, removing additional interferences by raising the pH and subsequent filtering, selectively adsorbing the isothiazolones in the sample desorbing the isothiazolones from the adsorbent, and comparing the absorbance of ultra-violet light of the desorbed sample to a standard of known concentration.