Colloidal liquid aphrons (CLAs) are surfactant-stabilised solvent droplets which have recently been explored for use in pre-dispersed solvent extraction (PDSE). In this work, the equilibrium partitioning of a microbial secondary metabolite, erythromycin, has been studied using both CLAs (formulated from 1% (w/v) Softanol 120 in decanol and 0.5% (w/v) SDS in water) and surfactant-containing, two-phase systems. The equilibrium partitioning of erythromycin was found to be strongly influenced by the extraction pH, and exhibited a marked change either side of the pK(a) of the molecule. A modified form of the Henderson-Hasselbach equation could be used as a simple design equation to predict the equilibrium partition cofficient (m(eryt)=C-org/C-aq) as a function of pH. For extraction experiments with dispersed CLAs where pH > pK(a), m(eryt) values as high as 150 could be obtained and the erythromycin could be concentrated by factors of up to 100. Experiments were also carried out in surfactant-containing, two-phase systems to determine the effect of individual surfactants used for aphron formulation on erythromycin partitioning. For extraction at pH 10 neither the Softanol (a nonionic surfactant) nor SDS (an anionic surfactant) had any influence on the equilibrium erythromycin partition coefficients. For stripping at pH 7, however, it was found that recovery of erythromycin from the organic phase decreased with increasing concentration of SDS, although again the Softanol had no influence on the equilibrium. The effect of SDS was attributed to a specific electrostatic interaction between individual erythromycin and SDS molecules under stripping conditions. The m(eryt) values and concentration factors achievable in the two-phase systems were considerably less than those for the PDSE experiments. The physical properties of the two-phase systems, ie density, viscosity, interfacial tension, etc, and the equilibrium distribution of the surfactants were also determined in relation to subsequent studies on the kinetics of erythromycin extraction.