Wang, MHS, Wang, LK, Wu, BC and Hu, GY (2022). Determination of humic substances (humic acid, fulvic acid & salts) in water by UV-visible spectrophotometry. In: “Environmental Science, Technology, Engineering, and Mathematics (STEM)”, Wang, LK, Wang, MHS, and Pankivskyi, YI (editors). Volume 2022, Number 4B, April 2022; pp.33. Lenox Institute Press, MA, USA. 10.17613/g0e5-wr96 ……….ABSTRACT: An analytical procedure has been developed for quantitative determination of soluble humic substances which are common agricultural and gardening products. The residual of soluble humic substances in surface water become the undesirable trihalomethane (THM) precursors to people and chronic toxic substances to aquatic plants and animals. Removal and analysis of soluble humic substances are important tasks of environmental researchers. The principle of humic substances is similar to that of tannin and lignin analysis except that the two analytical methods use different analytical instruments. The principle is that soluble humic substances, such as humic acid, fulvic acid and their salts, reduce the tungstophosphoric and molybdophosphoric acids to produce a grayish color up to at least 20 mg/L at UV wavelength 265-400 nm. The grayish color substance can be quantitatively measured with a UV-Visible Spectrophotometer. This publication is one of the authors’ memoirs.