Bioaccumulative and toxic perlfuorooctane sulfonate (PFOS) and perlfurooctanoic acid (PFOA) consist of fluorinated C8 backbones with a sulphonate and carboxylate group, respectively. Whilst strong C-F bonds deem PFOS biologically inert, its reactive sulphonate group can coordinate to different functional groups, which after polymerization, produces surface-active substances. For example, polyfluorinated sulfon-amides and telomer alcohols are widely used in textiles. However, incomplete polymerization will produce residual precursors such as perfluorooctane sulfon-amides (FOSEs, FOSAs) and acrylates (FTAcrs), which potentially degrade to PFOS and PFOA, respectively. Herein, we present a gas chromatography chemical ionization mass spectrometry (GC/CIMS) method capable of sensitive detection of four PFOS and three PFOA precursors. The PFOS and PFOA precursors covered in this study are included in the list of substances surveyed and revised by OECD (Organisation for Economic Cooperation and Development) since 2007.