AbstractCinnamic acids commonly found in forages were esterified to cellulose and tested for their inhibition of cellulose degradation by mixed cultures of rumen microorganisms in vitro. Concentrations of cinnamic acids used in the cellulose esterification procedure were 20–100 g kg−1, but measurement of actual ester concentrations by alkaline extraction were not possible for solka floc cellulose and ranged from 0–9.1 g kg−1 for Whatman No. 54 filter paper. Cinnamic acid esters significantly depressed cellulose digestion. For both cellulose sources, caffeic acid was the most inhibitory compound. Solka floc cellulose digestion was also inhibited by sinapic acid, whereas p‐coumaric and ferulic acids also depressed digestion of filter paper cellulose. The proportions of volatile fatty acids produced by fermentation of filter paper were not altered by cinnamic acid esters. Esterified cinnamic acids depressed cellulose digestion to a significantly greater extent than did free cinnamic acids. The data suggest that the natural form of cinnamic acids (ester‐linked to cell wall fibre) are inhibitors of microbial digestion at concentrations reported to occur in forages.