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Purpose -This paper aims to establish the acid concentration of finely dispersed droplets in hydrocarbon oils. Small quantities of aqueous sulphuric acid (H 2 SO 4 ) were found to be trapped within hydrocarbon shells, making them inaccessible for concentration evaluation by titration. Design/methodology/approach -Fourier transform infrared spectroscopy (FTIR) used in the attenuated total reflection mode (ATR; FTIR-ATR) was applied to study the reaction products of squalane, C 30 H 62 , and an API Group I base oil with various concentrations of aqueous H 2 SO 4 . Findings -The absorbance comparison usually used for estimating acid concentrations was found to fail when small quantities of acid are trapped in the reaction product. It was found that the peak shift and changes in absorbance found for various pure aqueous acid concentrations were useful to establish the remaining concentration of the trapped H 2 SO 4 . Originality/value -This paper fulfils the identified need to study acid dissociation-dependent peak shifts of H 2 SO 4 to find the acid concentration of finely dispersed droplets in hydrocarbon oils.