Routine monitoring of landfill leachates provides information concerning aqueous systems in which anaerobic microbiological processes influence water chemistry. Propionate and acetate are substrate and product, respectively, of metabolism by bacteria that have an obligate syntrophic relationship with sulfate-reducing bacteria. The stoichiometry of the bacteriological metabolic reactions indicates a 1:1 molar proportionality for acetate and propionate. This is observed for landfill leachates, consistent with the known biological control on their organic acid anion contents. Similar data for oil-field waters show the same 1:1 molar proportionality for reservoirs where bacterial sulfate reduction is known to take place, at temperatures up to about 95 °C, in contrast to the 3:2 proportionality (acetic acid:propionic acid) observed in higher temperature systems. These observations suggest that 1:1 molar proportionality for acetate and propionate may be characteristic of natural systems where anaerobic bacterial activity occurs, including bacterial sulfate reduction.