Abstract The in situ analysis of the distribution of electroactive solutes between aqueous and organic domains in microemulsions using electrochemical techniques is demonstrated. The usefulness of this method is illustrated by determining apparent distribution coefficients for substituted para-phenylenediamines [4-amino-3-methyl- N-ethyl- N-(β-sulfoethyl)aniline (PPD1) and 4-amino-3-methyl- N, N-diethyl aniline (PPD2)] in oil-in-water microemulsions. Cetyltrimethylammonium bromide (CTAB) and octadecyldimethyl betaine (C 18 DMB) were used as model surfactants. These apparent distribution coefficients are lower bounds to the true distribution coefficients. The partitioning of the PPDs into the organic domains is controlled by hydrophobic interaction and also by electrostatic attraction in the case of ionic surfactants. The distribution of K 4[Fe(CN) 6] in a CTAB microemulsion is also reported. The very small partitioning of ionic K 4[Fe(CN) 6] into the organic domain in the CTAB microemulsion is ascribed to electrostatic attraction by the cationic surfactant on the microemulsion droplet.