Lignin plays several fundamental roles in plants, but sometimes hampers the use of certain plants by man. With the aim of obtaining transgenic poplar trees better adapted to paper manufacture, modulation of the activities of two enzymes involved in the biosynthesis of lignin monomers was performed. Poplar lines over- or under-expressing (via antisense or co-suppression) caffeic acid/5-hydroxyferulic acid 0-methyl transferase (COMT) or cinnamyl alcohol dehydrogenase (CAD) were identified and characterized. Inhibition of COMT results in a dramatic change in the composition of lignin, leading to incorporation of a novel lignin unit, 5 hydroxyguaiacyl. Inhibition of CAD results in an increase in lignin extractability. These measurements were made on young stems. In order to assess thoroughly the effect of COMT or CAD down-regulation, lignin content and structure were monitored in these transgenic lines during a growing season and compared to lignin of control poplars. Older trees (2 year of age) were analyzed, in order to study the long term effect of the down-regulation of COMT or CAD activities. The characteristics of Kraft chemical and thermomechanical (TMP) paper pulps obtained from these poplars were determined. Transgenic poplars with modified COMT and CAD activities were obtained and the properties of the lignin characterized.