FeÀ TAML is a well-known green catalyst for bleaching and wastewater treatments. Quantitative analysis of FeÀ TAML is important to understand its activity and lifetime in these applications. However, because of its sub-parts-per-million dosage and the presence of interfering substances, it is very challenging to detect FeÀ TAML specifically and quantitatively at this level. To overcome these issues, we develop a quantitative assay for FeÀ TAML, which is based on its catalytic reaction with a blue dye (Reactive Blue 19) and H 2 O 2 . This assay is highly specific, and shows no interference for iron species. Moreover, the colorimetric assay can be carried out for fast screening of FeÀ TAML with a limit of detection (LOD) of 0.39 � 0.08 μg L À 1 . Finally, this assay was successfully applied to understand the stability of FeÀ TAML under various conditions, and monitoring the activity of FeÀ TAML during wastewater treatments.