In this study, a computational DFT study was performed to propose a new acid catalyzed mechanism to produce 5HMF from D-Fructose and D-Tagatose. The reactivity and selectivity towards 5HMF formation were analyzed, and the results revealed both saccharides present higher selectivity towards 5HMF with the first dehydration occurring on oxygen 2. Fructose seems to be more reactive than tagatose, although the dehydration process of the different hydroxyl groups on tagatose produces more unstable structures, which can undergo several side reactions. The new mechanism is proposed eliminating the tautomerization step and lowering the activation free energy of the second dehydration step in 21 kcal/mol.