A continuous microreaction process was set up to analyze and improve the "green" synthesis of 5-hydroxymethylfurfural (5-HMF) based oil the HCl-catalyzed dehydration of fructose in pure aqueous solutions. The process conditions were deliberately shifted to high temperatures and pressures (185 degrees C, 17 bar) and combined with a reduced reaction time of only 1 min to avoid extensive rehydration of the product. 5-HMF was obtained with 75 % selectivity and 54 % yield at 71 % fructose conversion. In comparison to published data, a 24 % increase in selectivity and a 21 % increase in conversion could be achieved. Moreover, by adding organic co-solvents and extraction agents to the reaction system, both 5-HMF selectivity and yield could be further enhanced to 85 and 82 %, respectively. However, the use of additional organic solvents cannot be considered as a green process anymore.