Abstract The acidity parameter in bile acid systems was re-examined in an attempt to unify the many contrasting results reported in the literature. Discrepancies originate not only through differences in experimental approaches but mainly through the peculiar behaviour of bile acids and their salts, which can be present in aqueous solution as monomers, or simple and/or mixed aggregates. The acidity (and the p K a values) of bile acid could also be affected by parameters which do not usually play a major role (e.g. hydroxy groups, far from the reaction centre). The microenvironment, where the bile acid is partitioned (e.g. inside a mixed micelle), also makes the carboxyl group less acidic than it would commonly be, considering only structural factors. When the system is made simpler, avoiding problems of solubility and self-aggregation (e.g. in mixed solvents), unconjugated and glycine-conjugated bile acids display acidity behaviour which matches their molecular structure.