This study focusses on the reversible/irreversible damage that selected phenolic compounds, released during steam-explosion pretreatment, mandatory for cellulose accessibility, causes on both stability and activity of a commercial cellulase (half-life = 173 h) during carboxymethyl-cellulose hydrolysis. Long-term experiments performed in continuous stirred UF-membrane bioreactors, operating at steady-state regime, in controlled operational conditions, allowed evaluating the inactivation-constant in the phenol presence (k(d)1) and after its removal (k(d)2) from the reactor feed. p-Hydroxybenzoic acid (1 and 2 g L-1) are the extreme limits in the inactivating effect with enzyme half-lives 99.02 and 14.15 h, respectively. The inactivation reversibility was assessed for vanillic acid, p-hydroxybenzoic acid, syringaldehyde, p-coumaric acid, being k(d)1 > k(d)2. p-Hydroxybenzaldehyde and protocatechuic acid irreversibly affected cellulase stability increasing its inactivation with k(d)2 > k(d)1. p-Hydroxybenzaldehyde, 1 g L-1, syringaldehyde, and vanillin, at 2 g L-1, had similar k(d)1 divided by k(d)2. (C) 2014 Elsevier Ltd. All rights reserved.