In the paper, Fourier transform infrared spectroscopy combined with Gaussian function linear fitting is used to analyze the structure of the viscoelastic scum formed during solvent extraction of sulfuric acid with trioctylamine (TOA). The results show that when the molar ratio of sulfuric acid to TOA is low in the system, it would form N center dot center dot center dot H-O- through TOA coupling with sulfuric acid or TOA coupling with water. The N center dot center dot center dot H stretching vibration is around 3400 cm(-1). With the increase of molar ratio of sulfuric acid to TOA, three peaks, near 2660 cm(-1), 2610 cm(-1), and 2535 cm(-1), appear in the infrared spectrum and they are assigned to N-H+ combining with water, N-H+ combining with O-S-, N-H+ combining with water and O-S- in the extraction system. The wave-number of C-N stretching vibration increases with the increase of molar ratio of sulfuric acid to TOA. (C) 2012 Elsevier B.V. All rights reserved.