The carbon isotope composition of leaf bulk organic matter was determined on the tropical tree Elaeis guineensis Jacq. (oil palm) in North Sumatra (Indonesia) to get a better understanding of the changes in carbon metabolism during the passage from heterotrophy to autotrophy of the leaves. Leaf soluble sugar (sucrose, glucose and fructose) contents, stomatal conductance and dark respiration, as well as leaf chlorophyll and nitrogen contents, were also investigated. Different growing stages were sampled from leaf rank −6 to rank 57. The mean values for the δ13C of bulk organic matter were −29.01 ± 0.9‰ for the leaflets during the autotrophic stage, −27.87 ± 1.08‰ for the petioles and −28.17 ± 1.09‰ for the rachises, which are in the range of expected values for a C3 plant. The differences in δ13C among leaf ranks clearly revealed the changes in the origin of the carbon source used for leaf growth. Leaves were 13C-enriched at ranks below zero (around −27‰). During this period, the ‘spear’ leaves were completely heterotrophic and reserves from storage organs were mobilised for the growth of these young emerging leaves. 13C-depletion was then observed when the leaf was expanding at rank 1, and there was a continuous decrease during the progressive passage from heterotrophy until reaching full autotrophy. Thereafter, the δ13C remained more or less constant at around −29.5‰. Changes in sugar content and in δ13C related to leaf ranks showed an interesting similarity of the passage from heterotrophy to autotrophy of oil palm leaves to the budburst of some temperate trees or seed germination reported in the literature. Copyright © 2009 John Wiley & Sons, Ltd.