he mixed stearic acid (SA)/sterol systems were used as sea spray aerosol mimics to get more insights into the al-terations in surface properties of aerosols induced by sterols. By means of surface pressure (π)–area(A) isotherms and polarization modulation-infrared reflection absorption spectroscopy (PM-IRRAS), the effect ofcholesterol (chol), stigmasterol (stig) and ergosterol (erg) on the lateral packing and chain conformation of SAmonolayer was explored. The fact that the excess areas of mixing of the mixed monolayers exhibit significant devi-ations from ideally mixedfilm proves that, the sterols are miscible withSA throughout all the monolayer composi-tions and surface pressures examined. The lift-off areas inπ–A isotherms were found to increase with increasingmole fraction of sterols, indicating that expulsive interactions exist between SA and sterols, which are more pro-nounced when the mole fraction of sterols is 0.7. In addition, the peak intensities ofνa(CH2)andνs(CH2)inIRRASspectra decrease with increasing sterols levels, which is consistent with ourfindings in theπ–A isotherms, thatthe addition of sterols leads to a looser chain packing in SA monolayer. The proportion ofgauchedefects in SA mono-layer induced by the sterols follows the order cholesterol stigmast ergosterol at a certain sterol level, as reflected by the decreasing peak intensities fνa(CH2)andνs(CH2). Consequently, the sterols generally give riseto considerable expanding effects on SA monolayer, which are particularly pronounced for stigmasterol and ergos-terol, suggesting that the additional alkyl side chains and double bonds of thesterols play a role on disordering SAmonolayer. The present study is likely to shed light on many boundary processes take place at the interface ofSSAs, in particular, transport processes of water and trace gases across the interface