Changes in pH and Eh in open cultures of sulfate-reducing bacteria consisting of various mixtures of broken or powdered beach shells (Pekcypoda) and a marine culturing medium were measured during a 40-day experiment. Sulfate reduction started almost immediately, and was brought about mainly through the action of bacteria of theDesulfovibriotype. In the liquid above the carbonate sediments, redox potential minima of about —200 mV were measured after only 5 days. After the first 5 days, the lowered redox gradually increased again to reach about —100 mV after 40 days. In the sediment itself, however, a lowering of the redox potential to values of —250 to —300 mV was observed, and still lowering at the end of the 40-day observation period. Addition of small amounts of Mg ions was not found to influence the change in redox potential, but addition of iron sulfate markedly increased (made more positive) the Eh. By plotting the change in pH/Eh conditions in the course of the experiment into a mineral stability diagram involving the biologically essential elements C-N-S-H-O in their respective stable compounds, information on the effect of sulfate reduction on carbonate sediments was obtained. A lowering of the redox potential of seawater below values of some —250 mV will lead to the precipitation of calcium carbonate and the production of methane gas instead of the otherwise more stable calcium and bicarbonate ions.