BACKGROUND: The hydrothermal conversion of glucose into lactic acid has been studied, in particular, the role and mechanism of action of three additives, Zn, Ni and activated carbon, for improving the conversion. RESULTS: Zn improves lactic acid yield more markedly than Ni or activated carbon. The highest lactic acid yield from glucose with Zn/Ni/ activated carbon reaches 55%. CONCLUSION: Zn may inhibit the decomposition of lactic acid, but also restrain the oxidation of Ni when Zn and Ni exist simultaneously. Ni plays a catalytic role and trace amounts of Ni2+ dissolved during the reaction improve the lactic acid yield. Activated carbon increases the contact area of the reactants. In addition, response surface methodology design (RSMD) results indicate that there are positive interactions between each of the added components, which contribute to the significantly improved yield of lactic yield from glucose. (C) 2016 Society of Chemical Industry