The kinetics for homogeneous hydrolysis of mannan is studied in a batch reactor at temperatures from 160 to 220 degrees C. A formate buffer ensures a pH of 3.8-4.0, measured at 25 degrees C. Samples are analyzed for oligosaccharides up to a degree of polymerization of 6 and also for the total amount of mannose after acid hydrolysis. A mathematical model with two reactions (1, random hydrolysis of the glucosidic bonds; 2, degradation of the reducing end of the molecule) describes accurately the time course of oligosaccharides. Optimized rate constants follow closely an Arrhenius relationship, with the degradation having a higher activation energy (140 kJ/mol) than the hydrolysis (113 kJ/mol). The mathematical model has the advantage that production of small molecules is independent of the initial chain-length distribution as long as the average initial chain length is some 5 times longer than the largest species measured. It can be applied to first-order depolymerization of other linear polymers with one link type in order to determine reaction rate constants or make predictions about molecular weight distribution on the base of known reaction rate constants.