In Quebracho colorado heartwood tannins are located in the lumina of most parenchyma cells, fibres and vessels. Formation and deposition of the tannins were studied by electron microscopy. In parenchyma cells phenolic compounds first occur at the inner and outer side of the membranes of vacuoles. Later, these vacuoles are completely filled with dark stained compounds and fuse together. From parenchyma cells the phenolic substances migrate into the lumina of fibres and vessels through pits. In pits they penetrate into the compound middle lamella (CML) and diffuse along the CML. Tannins were also found in the cell walls and in fissures within the CML and G-layer of tension wood fibres.