The colloidal properties of emulsans formed by incubations of Acinetobacter calcoaceticus RAG-1 on different carbon sources were studied. The apparent critical micelle concentrations (CMC) of the emulsans tested ranged from 25 to 58 mg/dm(-1). Surface and interfacial tensions of the solutions showed little dependence on pH between 2 and 10. In contrast, increasing the pH from 2 to 6.5 resulted in a substantial increase in their ability to effectively emulsify aliphatic hydrocarbons. Hexadecane-in-water emulsions were prepared having droplet sizes between 6 and 19 mu m. Many of the emulsions thus formed were found to be stable with respect to coalescence for several months. Certain structural features such as the total content of fatty acids and hydroxy fatty acids were found to have a significant effect on emulsifying activity. The maximum emulsifying activity occurred for emulsans containing about 460 nmol of total fatty acid per mg of emulsan (nmol mg(-1)). Emulsifying activity also showed a maximum at about 170 mol mg(-1)-emulsan of 2- and 3- hydroxy dodecanoic acids. For substituents having chain lengths greater than or equal to 15 carbonatoms, the emulsifying activity on hexadecane increased with their content up to 190 nmol mg(-1). On the other hand, for substituents having chain lengths of <15 carbonatoms, the emulsifying activity on hexadecane showed no obvious effect with their content up to 220 nmol mg(-1). A further increase in the shorter chain length fatty acids resulted in a decrease in emulsifying activity. Hence, a substrate-specific interaction between emulsans and the dispersed phase was observed.