TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The heterogeneous acid catalyst is acid resins, natural clay mineral, zeolites, supported acids such as silica impregnated with mineral acids, heat treated charcoal, metal oxides, metal sulfides, metal salts or mixed oxides.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Method: The reaction is performed at 50-300 (preferably from 100-250, especially 150-200)degreesC. The fructose and/or glucose-containing starting material is selected from starch, amylose, galactose, cellulose, hemi-cellulose, glucose-containing disaccharides such as sucrose, maltose, cellobiose, lactose (preferably glucose-containing disaccharides, especially sucrose or glucose). At least one solvent (S1) or diluents is present in addition to the organic acid or the anhydride in the reaction. The ratio of organic acid to solvent is 50-0.1 (preferably 20-1, especially 10-2). The reaction is performed in a continuous flow process (preferably fixed bed continuous flow process), and the residence time in the flow process is 0.1 second to 10 hours (preferably 1 second to 5 hours, especially 1 minute to 1 hour). The fixed bed comprises a heterogeneous acid catalyst. The continuous flow process is a reactive distillation or a catalytic distillation process. In addition to a heterogeneous acid catalyst, an inorganic or (halogenated) organic acid catalyst is added to the feed of the fixed bed or catalytic distillation continuous flow process. In the method, the liquid hourly space velocity is 1-1000 (preferably 5-500, especially 10-250, particularly 25-100). Preferred Components: The organic acid is a mono-carboxylic acid selected from (un)branched aliphatic acids, (un)branched unsaturated acids, (preferably 1-5C (un)branched aliphatic acids, most preferable formic acid, acetic acid, propionic acid, isobutyric acid, especially formic acid, acetic acid, particularly acetic acid or its anhydrides such as formic anhydride, acetic anhydride, propionic anhydride and/or isobutyric anhydride). The acid catalyst is selected from solid (halogenated) organic acids, inorganic acids, salts, Lewis acids, ion exchange resins and/or zeolites. The pKa of solid halogenated organic acid catalyst is less than or equal to the pKa of the organic acid. The acid is a solid Bronsted acid, solid Lewis acid. The solvent is selected form water, sulfoxides (preferably dimethyl sulfoxide and/or ketones (preferably methyl ethylketone, methylisobutylketone and/or acetone). EXAMPLE - In a continuous flow reactor, sucrose (10 mmol/l), dissolved in water/acetic acid/10% sulfuric acid4, and was reacted at 195degreesC with a residence time of 6-60 seconds and a flow rate of 10 ml/min (i.e. 3.33 ml/min/reactor). At 6 seconds, 5-hydroxymethylfurfural (HMF) and 5-acetoxymethylfurfural (AMF) selectivity was found to be greater than 90% at a conversion of 25%.