Two series of functionalized activated carbons have been prepared and used for the glucose to fructose isomerization reaction. Alkali earth chlorides and alkali halides have been chosen for the functionalization with the final goal to study the effect of cation and anion variation on isomerization activity. A part of the samples has been subjected to an activation procedure giving rise to the formation of new active sites of a distinct type and composition. The active site nature and density greatly influenced the reaction mechanism, giving rise to combined pathways catalyst with increased activity and fructose selectivity. The functionalization with MgCl2 resulted in a very stable and performant catalyst with an optimal fructose yield of 33% at 140 degrees C in only 20 min reaction time and during four cycles of reutilization.