In the present study, the disproportionation reaction of formaldehyde to methanol and formic acid was investigated in sub-critical water at 170-300 degrees C and 23 MPa. The cross-disproportionation of formaldehyde with formic acids to methanol and carbonic acid was also observed especially in the presence of an excess of formic acid. It was experimentally shown that methanol is a relative stabile molecule up to 270 degrees C and its presence in the reaction medium does not influence the cross-disproportionation and formic acid decomposition. In the experimental part, the rate of the HCHO consumption was found to be 27.9 (L/mol)(0.5) s(-1) exp(-58.5 kJ mol(-1)/RT) [HCHO](1.5). In fact the reaction of formaldehyde is very complex system of consecutive and parallel reactions. Some of these reactions are of major and some of minor importance in the range of reaction conditions. Concentrating of the major reaction steps, the reaction was described by a simple model consisting of only two reaction steps. The Arrhenius factors and the activation energy of these two main reaction steps are determined by fitting. Such a simple model describes the reaction of formaldehyde reasonable well in the temperature range up to 270 degrees C. At 300 degrees C, for a good description additional reactions steps in the model would be necessary. (C) 2012 Elsevier B.V. All rights reserved.