AbstractFour potential inhibitors, hydroxymethylfurfural, furfural, syringaldehyde and vanillin, were assayed over a range of concentrations and times to simulate the amounts that might be expected to accumulate when the hydrolyzed water‐soluble fraction from steam‐treated aspenwood was added in a fed‐batch mode to K. pneumoniae. Combinations of the individual materials resulted in a cumulative inhibitory effect. Prolonged incubation could alleviate some of the inhibition.