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Aqueous monosaccharide solutions including glucose or fructose have been hydrothermally treated in a closed system to form colloidal carbon spheres. In-situ Raman and ¹³C NMR have been used to quantify the intramolecular dehydration moiety, HMF, as an intermediate. An aqueous glucose solution forms a carbon sphere via an intermolecular dehydration route without forming HMF during initial hydrothermal treatment and followed by carbonization at 170−180 °C. However, an aqueous fructose solution initially forms HMF by intramolecular dehydration at 120−140 °C. Upon subsequent polymerization, microscopic carbon-containing spheres assemble to larger spheres, thereby generating a grain-like surface morphology. The carbon sphere contains a dense hydrophobic carbon core and a hydrophilic shell.