The sulfidogenic activity of two mesophilic sulfate reducing enrichment cultures was studied in H 2 -fed membrane bioreactors. The two enrichment cultures had different origins; one of them was a mesophilic and the other a psychrotolerant mesophilic culture. The operational temperatures of the reactors were gradually changed: for one the temperature was increased from 9 to 30°C and for the other it was decreased from 35 to 9°C. The specific sulfidogenic activities were 21–31, 52–53 and 57–92mmolSO 4 2− g VSS −1 d −1 at 9, 15 and 30–35°C, respectively. The sulfate reduction rate of the SRB stabilized to a lower level after the temperature was decreased. The percent electron flow to sulfate reduction was on average 24–32, 50 and 47–69% at 9, 15 and 30–35°C, respectively. The capability of mesophilic SRB to oxidize electron donor decreased as the temperature was decreased. The results indicate that starting of the reactor operation at 9°C resulted in higher sulfidogenic activity at sub-optimal temperatures and selective enrichment of the psychrotolerant species improved. The start-up of the reactor at 35°C resulted in decreased sulfidogenic activity as the temperature was decreased. This indicates that the operational temperature of bioreactors with mesophilic SRB can be decreased to 15–20°C and the sulfidogenic activity will decrease by 10–40%. Moreover, an operational temperature of 9°C seems to be close to the lower limit of active sulfate reduction for the mesophilic enrichment cultures used in this study.