Light Cycle Oil (LCO) from Fluid Catalytic Cracking (FCC) is becoming an important source for gas oil production, although deep hydrotreatment steps are needed to face its poor ignition properties. While hydrogenation of poly-aromatics is insufficient to significantly increase the cetane number of the LCO mixture, the conversion of these hydrocarbons into linear alkanes (2-ROPs) and alkyl-cycloalkanes/alkyl-benzenes (1-ROPs) through selective ring opening (SRO) is highly desirable, leading also to an improvement of the fuel cold-flow properties and a decrease of soot formation. SRO consists in reducing the number of rings while retaining the carbon number of the starting molecules. Besides SRO, competing parallel and consecutive reactions such as hydrogenation/dehydrogenation, cracking, and skeletal isomerization (Sk-Iso) affect the final fuel composition and quality. This complex reaction network makes difficult the complexity of the analytical investigation of the process, even when model molecules (such as decalin or tetralin) are involved. As an example, more than 350 C10 molecules were detected by conventional gas chromatography (GC) downstream a decalin hydroconversion test, with very short retention time differences between GC peaks [1]. The present study investigates the separation and identification of the products in such complex reaction mixtures by means of several GC techniques, including two-dimensional GC-MS (Figure 1). While these analytical aspects are often neglected in the literature [2], this work points out the added value of accurate chemical compound identification, as a crucial step to gain insight into the reaction mechanism. In turn, this provides important information on the nature of the relevant catalytic functions in bifunctional systems. In the present case study of SRO on conventional transition metal sulfides (NiW supported on amorphous silica-alumina), the acidic or hydrogenolytic nature of the process (i.e., involving acidic or metallic catalytic sites) is investigated by evaluating the nature of ring opening products. 1] S. Rabl, , PhD thesis, University of Stuttgart, 2011 [2] L. Piccolo, S. Nassreddine, G. Toussaint, C. Geantet, J. Chromatogr. A 1217 (2010) 5872.