The salinity and sulfate concentrations of microbial mats collected from hypersaline salterns were mainipulated to examine the effects of low sulfate concentrations and lowered salinity on carbon metabolism. As sulfate was slowly removed from the mats, methane production and flux increased, with highest fluxes and concentration in mats at lowered salinity (35 ppt) and low sulfate concentrations. The δ13C values of bulk particulate organic carbon (POC) ranged from about -10 to -12 &perthou;, similar to what had been observed previously for these cyanobacterial mats. In mats with higher sulfate concentrations, pore water profiles of dissolved inorganic carbon (DIC) δ13C values decreased with depth. However, in the mats with lowered sulfate, the DIC δ13C values increased substantially, from about -1 &perthou; in the overlying water to +12 &perthou; by 20 mm depth. Although the increase in DIC δ13C values is consistent with biogenic methanogenesis, the measured methane concentrations in these mats were not great enough to be the sole cause of the increase. These positive isotopic values, as well as the higher acetate concentrations observed in the low-sulfate mats, are also consistent with the occurrence of acetogenesis.