Cinnamoyl-CoA reductase and cinnamyl alcohol dehydrogenase were purified to apparent homogeneity from poplar stems (Populus euramericana) and their main properties were studied. Only one form was identified for each enzyme. The reductase corresponded to one polypeptide of molecular weight 36 000 and the cinnamyl alcohol dehydrogenase was constituted of two identical subunits of molecular weight 40 000. These characteristics are in agreement with most of the data obtained for the same enzymes isolated from other plants. The two reductive enzymes are inhibited by thiol reagents and a metal chelator 1,10-phenanthroline. The isoelectric point of the reductase (pH 7.5) and of the dehydrogenase (pH 5.6) were determined by chromatofocusing. The cinnamoyl-CoA reductase exhibit a decreasing affinity towards feruloyl-CoA, sinapoyl-CoA and p-coumaroyl-CoA. The cinnamyl alcohol dehydrogenase, which catalyses the reduction of the three cinnamaldehydes, exhibits its highest efficiency towards coniferaldehyde. In spite of differences in the monomeric composition of lignins from xylem and sclerenchyma the reductive enzymes isolated from these two lignified tissues exhibit the same substrate specificity. Consequently, they do not play an important role in the qualitative control of lignins in poplar tissues.