The aim of this investigation was to survey the reactions taking place during heat-induced yellowing of a TCF-bleached birch kraft pulp bleached according to the sequence O Q P Q (PO). This was done by determining the metal profiles in pulp and aqueous pulp extracts before and after accelerated ageing of pulp. It was concluded that low-molecular substances were formed during heat treatment of the pulp. It was further shown that hexenuronic acid (HexA) groups were hydrolysed during the ageing and that the destruction of HexA significantly lowered the chelating properties of the pulp, implying a liberation of metal ions. Two different procedures to isolate the coloured compounds in an aqueous extract of aged pulp are reported. Analysis of the separated strongly coloured material revealed that manganese and iron as well as some dicarboxylic acids were enriched. It was also concluded that if iron ions were removed without hydrolysing HexA, the brightness stability could be improved. Finally some observations and conclusions from previous work are discussed.