This paper reports a systematic investigation on the primary decomposition mechanism and kinetics of cellobiose under hydrothermal conditions at 200-275 degrees C and a wide initial concentration range of 10-10,000 mg L-1. Isomerization of cellobiose to cellobiulose (glucosyl-fructose) and glucosyl-mannose dominates the primary reactions of cellobiose decomposition, contributing to 71-93% of cellobiose decomposition depending on reaction conditions. In contrast, cellobiose hydrolysis to glucose makes only limited contributions (6-27% depending on reaction conditions) to the primary decomposition of cellobiose. This indicates that hydroxyl ions have a more significant effect to catalyze the isomerization reactions to produce cellobiulose and glucosyl-mannose. The catalytic effect of hydronium ions is weak probably because of the high affinity of hydronium ions for water molecules, which reduces the availability of hydronium ions for catalyzing the hydrolysis reaction. At increased temperatures, the affinity of hydronium ions for water molecules decreases because of the weakened hydrogen bonds in water, leading to an increase in the selectivity of the acid-catalyzed hydrolysis reaction. A higher initial cellobiose concentration also promotes hydrolysis reaction due to the formation of acidic products at the early stage of cellobiose decomposition. As a result of the reduced molar ratio of ion product to cellobiose, the activation energies of both isomerization and hydrolysis reactions increase with an increase in initial concentration, leading to an increase in the apparent activation energy of cellobiose hydrothermal conversion.