Wang, MHS, and Wang, LK (2022). Modified standard colorimetric method for determination of lignin and tannin in industrial waters. In: “Environmental Science, Technology, Engineering, and Mathematics (STEM)”, Wang, LK, Wang, MHS, and Pankivskyi, YI (editors). Volume 2022, Number 4, April 2022; 23 pages. Lenox Institute Press, MA, USA. …………… ABSTRACT: In tannin and lignin determination using the standard colorimetric method, if the industrial water contains high concentrations of calcium ions, precipitates are formed at the same time of the color development., therefore, causing interferences at the time of obtaining instrument readings. This publication introduces a modified Standard Method which adds the extra analytical steps of: (a) filtration of the test water sample with 0.45 micron pore-size membrane filters immediately after the completion of the color development, so accurate instrument readings can be obtained; (b) preparation of calibration curves when using instrument; and (c) preparation of Standard Nessler tubes for color comparison in case an instrument is not available. Since the analytical procedures introduced in this publication is meant to be a modified “Standard Method”, the authors include the exact words in original “Standard Methods 5550 Tannin and Lignin” , but add the authors’ recommended extra analytical procedures for the convenience of the readers.