Eucalyptus globulus heartwood and sapwood were chemically characterized and their behaviour was investigated during isothermal kraft pulping at 130, 150 and 170 degrees C regarding mass loss and lignin removal. Heartwood contained more extractives than sapwood (9.8 vs 3.9%) but similar lignin content (23.5 vs 24.3%). Pulping yields of heartwood were lower than sapwood's by 8% at 130 degrees C, 4% at 150 and 170 degrees C after 180 min pulping time. Lignin removal was only slightly lower in heartwood in relation to sapwood (residual lignin of 2.0 and 1.1% after 180 mm at 170 degrees C, respectively). The delignification rate at 170 degrees C increased rapidly during the first 25 min until a constant residual lignin after 35 min. A similar behaviour occurred at 150 degrees C, but with a lower initial decrease rate in lignin content and stabilizing only after 95 min, while at 130 degrees C lignin removal was approximately linear with time without reaching a constant value during the tested period. E. globulus heartwood compared unfavourably to sapwood in kraft pulping owing to lower yields, mainly due to its higher content of extractives.