Critical micelle concentrations (cmc＇s) of macroemulsion (styrene + SDS + water) and miniemulsion (styrene + DMA + SDS + water + NaHCO3) solutions are determined by a conductance technique. A four-terminal cell and an impedance spectrometer are employed in this work. An equivalent circuit of the sample cell is determined, and the solution resistance is obtained from the impedance plane plot. Cmc＇s of the macro-/miniemulsions are then determined from the slope break in the conductivity-surfactant concentration (kappa-C) diagram. The cmc of the macroemulsion is found to be only 10% greater than that of the sodium dodecyl sulfate (SDS) surfactant solution, whereas the cmc of the miniemulsion is about five times that of the SDS solution. The surface concentration of SDS at the styrene/water interface is evaluated from the equilibrium interfacial tension data, measured by a video-enhanced pendant drop tensiometer, and the micelle ionization degree, evaluated from the slope change in kappa-C plot. A video-enhanced optical microscope is utilized to evaluate the size distribution of the emulsified monomer oil droplets. A mass balance on SDS molecules is established for the macro-/miniemulsion systems. The increase of cmc is primarily due to the adsorption of SDS onto the styrene/water interface of these tiny monomer droplets in the bulk phase of the macro-/miniemulsion solutions.