In this work, synthesis of 5-hydroxymethylfurfural (5-HMF) from lignocellulosic biomass derived hexoses, fructose in our case, was developed in two-phase supercritical CO2-water systems. Conditions of reactions were above 10 MPa and 120 degrees C, i.e., the two-phase reacting mixture can be regarded as a subcritical water-supercritical CO2 system. From kinetic experiments and their modeling, it was possible to assess the effect of CO2 as a potential reversible acid catalyst. The kinetic model developed in this study considers also the catalytic contribution of acid byproducts, as proton providers, for the reaction of dehydration of fructose into 5-HMF. Selectivity toward S-HMF was improved with increasing pressure to reach a maximum of 80 mol % after 3 h of reaction at 150 degrees C and under 25 MPa of CO2, as only 60% was achieved without the use of pressurized CO2.