AbstractThis paper describes the development of a simple UV spectroscopic method for determination of silicon content in black liquor. The method is based on the fact that Si (IV) can react with ammonium molybdate to form Si-Mo heterophony acid in an acidic condition (pH=4). The absorption peak of α-Si-Mo heterophony acid is 340 nm. To avoid the compensation of the instrument, 360 nm is used for spectroscopic quantification. A certain time (20 min) for the reaction is necessary. Moreover, it was discovered that 480 nm could be used to develop a dual-wavelength method to account for the spectral interference from dissolved lignin in black liquor. This method is simple, rapid, sensitive, accurate, and has the potential for on-line applications.