The thermal decomposition of wood, conducted in the absence of oxygen, includes so-called primary and secondary reactions, where secondary reactions are regarded as those involving previously formed decomposition products. This study uses a pyrolysis gas chromatograph-mass spectrometer, Py-GC/MS, to investigate the degree to which secondary reactions affect the composition of the volatile pyrolysis gases. Samples were Monterey Pine (Pines radiata) and were held in either open crucibles or sealed capsules. For specified heating profiles ranging from 240 to 700 degrees C, volatiles escaped soon after they evolved in the open crucibles but in the sealed capsules they were retained at high pressures for long residence times. Differences were observed over the entire range of treatment temperatures, which suggests that secondary reactions occur concurrently to primary reactions and implies that they are more dependent on vapour-phase concentration than on temperature. In comparison to the open crucible experiments, the sealed capsule experiments produced more low molecular weight pyrolysis products and more long chain alkanes, alkenes and methyl ketones, but at the expense of extractives, carbohydrate and lignin derivatives. The sealed capsules also had less oxygen in the condensable fraction, characterised by the presence of alkyl substituted compounds. Instead the open crucible experiments contained methoxyl substituted compounds. These differences are all characteristic of secondary reactions. This study shows that controlling the extent of secondary reactions is an invaluable tool for optimising pyrolysis.