The formation of emulsions in the reversed micelle extraction of proteins from filtered broth is a major problem which can significantly increase the cast of downstream separation. In this work the effect of some aqueous phase constituents such as buffer salts, other added cations, phospholipids and five molecular-weight fractions (fractionated using ultrafiltration membranes) of a fermentation broth of Bacillus stearothermophilus on emulsion formation and stability of a water-AOT-isooctane reversed micelle extraction system were examined. The parameters used for the determination of emulsion stability were the settling time and interfacial tension, and this varied from 33mNm(-1) for pure water-isooctane to around 1mNm(-1) for high AOT concentrations (>100mmoldm(-3)). K+ ions of the buffer solution, as well as K+ ions and Na+ ions from added salts had a destabilising effect on a water-AOT-isooctane emulsion. This effect was found to be greatest for the K+ ions, possibly due to their smaller hydrated radius compared with the other ions, and reduced the phase separation time with 50mmol dm(-3) AOT from 500min to less than 5min. The phospholipids phosphatidylcholine and phosphatidylinositol were also found to be effective demulsifiers, with phosphatidylcholine having a slightly greater effect. Albumin and beta-galactosidase both exhibited a strong emulsifying effect on a surfactant-free water-isooctane system, but were capable of strongly reducing emulsification in the presence of AOT. High molecular weight fractions (>30M) of the B stearothermophilus fermentation broth had a much stronger tendency to form emulsions than MW fractions <30kDa, while the 30-50kDa MW fraction had the best demulsifying effect, reducing the separation time to 751min. Demulsifying properties of the components tested were attributed to the competition of these compounds with AOT for control of the aqueous-organic interface leading to a less stable mixed interfacial layer.