In nature, it has long been assumed that p-hydroxycinnamyl alcohols occur exclusively in their trans (E) stereoisomeric form. However, in beech bark (Fagus grandifolia Ehrh.) only the corresponding cis (Z) monomers were found. Experiments with beech bark and 2-¹⁴C-labelled E ferulic acid, E coniferaldehyde, and E coniferyl alcohol revealed that these compounds were efficiently incorporated into both E and Z isomers of coniferyl alcohol. On the other hand, low incorporation of 2-¹⁴C-labelled Z ferulic acid indicated that this isomer was not a preferred substrate. Our results suggest that Z coniferyl alcohol is produced by E + Z isomerization of the corresponding E isomer. Experimental evidence excluded the possibility of this unusual isomerization occurring by either chemical or photochemical means; instead it is concluded that isomerization is enzymatically mediated by a novel isomerase. / M.S.