Pulping kinetics of extractive-free wood from jack pine trees sampled from the Fort Frances, the Atikokan and the Thunder Bay districts of Ontario, Canada, were investigated. Wood samples were cooked at three different temperatures: 140 °C, 155°C and 170 °C with a white liquor concentration of 32 g/L sodium hydroxide and 10.4 g/L sodium sulphide (25% sulphidity). The results indicated that the average activation energy for pulping of trees from the Fort Frances, the Atikokan and the Thunder Bay districts were 91.0, 81.0 and 81.4 kJ/mol, respectively. The differences of activation energy for pulping between sapwood and heartwood were insignificant. Activation energy for pulping of wood increased from top to bottom of the tree.