This European Standard specifies a method for the quantitative determination of the urease inhibitors N-(n-Butyl)thiophosphoric acid triamide (NBPT, CAS-No. 94317-64-3) and N-(n-Propyl)thiophosphoric acid triamide (NPPT, CAS-No. 916809-14-8) content in urea based fertilizers using high-performance liquid chromatography (HPLC).