Abstract Evidence showed that acetyl groups introduced during acetic acid delignification was a primary cause of the poor enzymatic digestibility of acetic acid pulp. The inhibition by acetyl groups could be removed by saponification. Acetyl groups might inhibit the enzymes by interfering with the productive binding (hydrogen bonds) between cellulose and the catalytic domain of cellulases, by affecting the binding of CBD to cellulose, or by increasing the diameter of the cellulose chain.