This review discusses various interactions between organic compounds, brought into the lead–acid battery via the separator, and their subsequent effect on battery performance. Historically, the interrelationship started with certain ‘expander’ actions on the lead morphology due to lignins, which leached out of the wooden separators of that time. Synthetic separator materials did not show this effect, but gained acceptance as they were far more stable in the hostile battery environment. The partially hydrophobic character of synthetic separators has been overcome by organic surfactants. Other organic compounds have been found to improve further the stability of separators against oxidation. Special organic molecules, namely aldehydes and ketones, have been identified to retard, or even suppress, the adverse effects of metals such as antimony, and thus prolong the cycle-life of traction batteries in heavy-duty applications or reduce water loss from automotive batteries. Knowledge about these interactions has opened ways to improve separators.