In situ mass spectrometry analysis was conducted for the hydrothermal decomposition of mannitol at different temperatures and residence times in a batch reactor. The temperature was varied in the range of 140-220 degrees C, and the residence time was varied between 5 and 20 min. The products of mannitol decomposition included furfural, 5-hydroxymethylfurfural, phenol, formic acid, and levulinic acid. At high temperatures, the mass spectrum showed peaks at m/z 46 and 116 that corresponded to formic acid and levulinic acid, respectively. Conventional high-performance liquid chromatography could not detect these acids. Quantification of these products and the rate constants for the mannitol decomposition network were also obtained. The reaction rate coefficients were determined by fitting the experimental results.