The polarizing microscope was used to study the structure of aqueous solutions of petroleum sulfonates in the composition range planned for injection. All the surfactants studied showed a basic pattern of transformation between one structure and another over a relatively narrow range of salinities. As the salinity where this transformation occurs increases, so does the optimum salinity with a given oil, the condition of lowest interfacial tension. A polarized light box is being: developed to permit rapid determination of solution structure without the microscope since information on structure may be useful in the early stages -0f surfactant selection. Solutions containing a well-characterized surfactant, sodium dodecyl sulfate, were found to have the same basic structure pattern as the sulfonates. Hence, further experiments with this simple material should provide information relevant to the more complex petroleum sulfonates. The microscope was used to observe the dynamic contacting process between a surfactant solution and oil. Equilibration proceeds more rapidly at high salinities, mainly because of the spontaneous emulsification which occurs under these conditions. The ultracentrifuge was used to study microemulsions containing a conventional petroleum sulfonate. Results were similar to previous results for synthetic sulfonates except in a region where interfacial tension and solubilization results for the conventional sulfonate were anomalous. Progress was made in development of a theory to predict drop size in microemulsions, an important property influencing both phase behavior and interfacial tension.