We have investigated the impact of the brown-midrib bm5 mutation on lignins and on p-coumaric acid and ferulic acid ester-linked to maize (Zea mays L.) cell walls. Lignified stalks or plant aerial parts (without ears) collected at grain maturity were studied in three genetic backgrounds. Relative to the control, bm5 mutants displayed lower levels of lignins and of p-coumarate esters but increased levels of ferulate esters. Thioacidolysis revealed that bm5 lignins display an increased frequency of free-phenolic guaiacyl units. More importantly, thioacidolysis provided unusual amounts of 1,2,2-trithioethyl ethylguaiacol, a marker compound diagnostic for the incorporation of free ferulic acid into lignins by bis 8-O-4 cross-coupling. As the resulting acetal bonding pattern is a chemically labile branch point introduced in maize lignins by the bm5 mutation, this alteration is prone to facilitate the delignification pretreatments used in the cellulose-to-ethanol process