The complexation constants of Ni 2+ and Co 2+ with pyridine-2-azo-p-dimethylaniline (PADA) were determined in sodium bis(2-ethylhexyl)sulfosuccinate (AOT)/isooctane/water microemulsions at 25 °C. The results show that the complexation constants depend on the microemulsion composition. In all cases, the values of the apparent complexation constants are greater than those obtained in bulk water. To carry out a quantitative interpretation of the experimental results, we propose a complexation scheme that considers the microheterogeneous character of the microemulsion. It was found that the PADA is distributed between the continuous medium and the interface of the latter, whereas the Ni 2+ or Co 2+ ions are distributed between the aqueous microdroplet and the interface. If we consider an ionic exchange equilibrium between the counterion (Na + ) of the AOT and the reactive cation (Ni 2+ or Co 2+ ), it is possible to obtain the true complexation constants at the interface of the microemulsion. The values obtained for the true complexation constants decrease as the water content of the microemulsion increases and are always lower than the values obtained in bulk water. The interaction of interfacial water with the surfactant headgroup in the microemulsion causes an increase in the electronic density on the oxygen atoms of water and a consequent increase in the interaction H 2O‚‚‚M 2+ , which in turn leads to a greater stabilization of the ion with respect to that in bulk water. As the water content decreases, this interaction becomes stronger but the quantity of water molecules available for hydration decreases. This situation results in destabilization of the ion and an increase in its complexation capacity.