The Retro-Aldol Condensation (RAC) of sugars is a key reaction in the upgrading of lignocellulosic biomass. The RAC of glucose yields the key intermediate glycolaldehyde for the production of ethylene glycol. However, the reaction media under which it may occur is complex as many side transformations occur, such as the epimerization and the isomerization. Sodium tungstate and molybdate (Na2WO4 and Na2MoO4) or homopolyanions such as ammonium metatungstate and heptamolybdate (AMT and HMA) are active homogeneous catalysts for the RAC of sugars. These species are also catalytically active for epimerization, but remarkably do not promote the isomerization into fructose. A hypothesis in the literature proposed that RAC and epimerization have common mechanistic steps and possibly same active species. However, a clear understanding of the different reaction mechanisms at a molecular scale has not been yet reached. Hence, a fundamental research work was conducted in the present thesis in order to disclose the nature of the genuine active species controlling the selectivity towards these transformations. A first study aimed at probing the possible metal-carbohydrate complexes in aqueous solutions by means of ex-situ NMR and XAS measurements in combination with DFT modeling. New structures for sugars-metal complexes in solution have been proposed, that could not have been resolved without this multi-technique approach. The catalytic behavior of these complexes was then investigated by means of a kinetic study considering different parameters. Several models of activated complexes were then calculated by DFT for different reactions. It is concluded that RAC and epimerization are catalyzed by different active species. A new mechanism is proposed for sugars epimerization over tungstate and molybdate catalysts. Other reaction pathways were computed for RAC for which the activation barriers showed a good correlation with the experimental activation energies.