The production of lactic acid has increased owing to growing polymer markets, increased demand in the chemical sector and many applications in the food industry. Biomass hydrothermal decomposition is potentially a method for lactic acid production. To obtain a higher yield of lactic acid, the influence of metal ions (Zn(II), Ni(II),Co(II) and Cr(III)) on biomass decomposition in sub-critical water (T = 300 degrees C) was investigated and the catalyst function in the complex reaction network of biomass degradation was discussed. In comparison with a non-catalytic process, the addition of 400 ppm Ni(II) catalyst increased the yield of lactic acid from 3.25% to 6.62% at 300 degrees C and 120 s for microcrystalline cellulose. The lactic acid yield for glucose was 9.51% for 400 ppm Co(II) catalyst at 300 degrees C and 120 s. In the case of Cr(III) and Ni(II), the conversion of maize straw, sawdust and rice husk first increased, and then decreased from 0 to 800 ppm. For catalyst Cr(III), Zn(II) and Ni(II), the conversion of wheat bran indicated a decreasing trend. Transition metal ions have a great influence on raw materials conversion to lactic acid. In the conversion of pyruvaldehyde to lactic acid, the ionic catalyzed Cannizzaro-type reaction would take place. (C) 2007 Society of Chemical Industry.