Summary The aim of the present study was to describe the spatial distribution of some chemical and structural changes in the transition zone between sapwood and heartwood of Scots pine (Pinus sylvestris L.) in order to get a better understanding of heartwood formation. The content of pinosylvins was measured with FT-(NIR) Raman spectroscopy and gas chromatography (GC). Determinations of resin acids, free fatty acids, triglycerides and steryl esters were also made with GC. Lignification, nucleic disappearance and occurrence of starch in ray parenchyma cells were studied with light microscopy. The measured chemical and structural changes occurred in a narrow area of approximately one growth ring adjacent to the heartwood border. In this growth ring, pinosylvins, resin acids and free fatty acids increased at almost the same position, whereas triglycerides decreased. The structural changes occurred after the chemical changes had started and may have been caused by these changes.