Solute-micelle association constants and partition coefficients between stationary, aqueous, and micellar phases have been determined for a group of benzene derivatives and polycyclic aromatic hydrocarbons with sodium dodecyl sulphate and hexadecyltrimethylammcnium bromide by Micellar Liquid Chromatography (MLC) with an octylsilica column using micellar mobile phases in absence and in the presence of organic modifiers (methanol, n-propanol, and n-butanol). The retention mechanism of these compounds in the chromatographic system has been studied by comparing experimental capacity factors and selectivity coefficients with those theoretically calculated assuming a direct transfer mechanism. When the surfactant concentration in the mobile phase is increased, a tendency to a change from a three-partition equilibrium mechanism to a direct transfer of solutes from micellar to stationary phase is observed for both surfactants. This change was favored when: (1) the hydrophobic character of the solute increased, (2) CTAB was used as surfactant, and (3) the polarity of the aqueous mobile phase was increased. For highly hydrophobic compounds experiencing a direct transfer mechanism, selectivity coefficients for a pair of solutes can be predicted from the ratio of two stationary-micellarpartition coefficients.