Abstract In the waters produced by oil and gas wells, the presence of anions of weak organic acids, mainly acetates, has been reported for a long time. In routine water analyses however, they are generally not differentiated from bicarbonates, and this is shown to induce considerable errors in the appreciation of the pH of production waters under CO2 pressure, and of their degree of calcareous saturation. This task requires indeed a complete water analysis, in which the contents in calcium, bicarbonate and acetate (or equivalent anions) must be explicitly mentionned. Acetates, as bicarbonates, contribute to the alkalinity of waters. Nevertheless, pH remains practically controlled by the bicarbonate content only, except if the latter is greatly exceeded by the acetate content. On the other hand, for a given pH, the presence of acetates increased drastically the diffusion limited current of the cathodic reduction of H+ ions. Therefore, despite their low effect on acidity, acetate might have a much more important effect upon the CO2 corrosion of steels than was at first imagined.