Hydrogen-producing mixed cultures were subjected to a 48-h downward or upward temperature fluctuation from 55 to 35 or 75 degrees C. Hydrogen production was monitored during the fluctuations and for three consecutive batch cultivations at 55 degrees C to evaluate the impact of temperature fluctuations and bioaugmentation with synthetic mixed culture of known H-2 producers either during or after the fluctuation. Without augmentation, H-2 production was significantly reduced during the downward temperature fluctuation and no H-2 was produced during the upward fluctuation. H-2 production improved significantly during temperature fluctuation when bioaugmentation was applied to cultures exposed to downward or upward temperatures. However, when bioaugmentation was applied after the fluctuation, i.e., when the cultures were returned to 55 degrees C, the H-2 yields obtained were between 1.6 and 5% higher than when bioaugmentation was applied during the fluctuation. Thus, the results indicate the usefulness of bioaugmentation in process recovery, especially if bioaugmentation time is optimised.