During the sulfonation process of the linear alkylbenzene (LAB) other products like sulfones and anhydrides are also formed in addition to the linear alkylbenzene sulfonic acid (HLAS). Most of them are transformed in different degrees during the aging and hydrolysis steps of the overall sulfonation process [Moreno et al. (2003) J Surfactant Deterg 6(2):137–142]. The use of HPLC technique since 1988 and earlier the use of high temperature gas chromatography (HT-GC) enable the separation and quantification of all the components of the free oil (FO) of the sulfonic acids—anhydrides; LAB and sulfones. The purpose of this paper is to review the current knowledge about the free oil analysis and to discuss a new analytical procedure based on the HT-GC to quantify the free oil content. We will compare the results obtained using this new method with other analytical procedures (traditional gravimetry) and near infrared (NIR) spectroscopy.