We explored the isomerization of d-glucose into d-fructose using the simplest possible base catalyst, aqueous NaOH, to maintain a constant pH value during the reaction. Under the applied mild conditions (T 50–90 °C, pH 9.5–11.5), yields of d-fructose of up to 31% were observed. Selectivity-conversion plots were not significantly influenced by variation of the temperature, pH value or substrate concentration. A reaction network for kinetic modelling includes d-glucose-d-fructose interconversion, co-production of d-mannose and d-allulose (also known as d-psicose) as well as decomposition paths after deprotonation of the hexoses. All four hexoses were employed as substrates in the isomerization. Thermodynamic ionization constants of the saccharides were measured by means of potentiometric titration. In the kinetic studies, pH-independent rate constants as well as activation energies were determined. The obtained kinetic and thermodynamic results as well as selectivity-conversion correlations present a useful benchmark for soluble and solid base catalysts.