Analysing a complex mixture of unknown macromolecules and low molar mass compounds is often challenging in GPC/SEC. A disadvantage of the most commonly used detector, the very universal but unspecific refractive index (RI) detector, is that this detector only provides concentration information and no information about the chemistry. In addition, various positive and negative peaks besides the pure sample peaks in a GPC/SEC run are normal. However, only when the chemical structures of the polymer components are known can sophisticated separation techniques, such as GPC/SEC, liquid adsorption chromatography (LAC) or liquid adsorption chromatography under critical conditions (LACCC) be properly adapted. Also, advanced detection such as on-line light scattering or triple detection requires the knowledge of the chemistry, since these techniques need specific sample related constants (e.g., the refractive index increment dn/dc). Therefore peak identification or substance identification can be very important in GPC/SEC, especially when establishing new methods. There are several possibilities for GPC/SEC users to identify peaks or to learn more about sample chemistry.