Fourier transform infrared spectrum (FT-IR) combined with Gaussian function linear fitting is applied to analyze the aggregation structure in the system of oleamide mixed with sulfuric acid aqueous solution. The possible aggregation structure is proposed according to the shifts of hydroxyl peaks and the changes of their percentage areas. Results show that the interaction modes between sulfuric acid and oleamide vary with the different proportions of two ingredients. When the molar ratio of sulfuric acid to oleamide is low, the major forms of water existing in the system are bulk-like water and the water combining with N+-H as N+-H...O-H-2. With the increase of the molar ratio of sulfuric acid to oleamide, N+ forms ion-pair complex through attracting sulfate radical. That may be responsible to the scum-like aggregation. (C) 2015 Published by Elsevier B.V.