This article finds that the transcript profile of the laccase gene ChLAC8 parallels the accumulation of C-lignin during seed coat development. Recombinant ChLAC8 oxidizes caffeyl and sinapyl alcohols, generating their corresponding dimers or trimers in vitro, but cannot oxidize coniferyl alcohol. The authors propose a basis for this substrate preference based on molecular modeling/docking experiments. Suppression of ChLAC8 expression led to significantly reduced C-lignin content in the seed coats of transgenic Cleome plants. Feeding of 13C-caffeyl alcohol to the Arabidopsis (Arabidopsis thaliana) caffeic acid o-methyltransferase mutant resulted in no incorporation of 13C into C-lignin, but expressing ChLAC8 in this genetic background led to appearance of C-lignin with >40% label incorporation. These results indicate that ChLAC8 is required for C-lignin polymerization and determines lignin composition when caffeyl alcohol is available.