A conductivity meter is an inexpensive instrument that can easily be installed in polymerization reactors. This instrument can be used to monitor ionic species without time-consuming calibrations. A probe is inserted into the media, providing hi situ measurements of conductivity in real time. For emulsion polymerization reactions, the conductivity meter can respond to changes in the ionic surfactant concentration, allowing the determination of surfactant dynamics in the media. The surfactant concentration can then be related to the changes in the surface area of the polymer particle phase, which can be linked to nucleation or coagulation phenomena. In this study, a conductivity meter was coupled to a calorimetric reactor to provide in situ and online measurements of conductivity during the emulsion polymerization of styrene, with sodium dodecyl sulfate as an anionic surfactant and with potassium persulfate as a free-radical initiator. A semiempirical model was built to describe the conductivity signal as a function of the latex composition and the reactor temperature. The model was inverted and combined with the available conductivity signal, conversion, and temperature measurements and was able to accurately predict the number of polymer particles in the latex and the surfactant concentrations in the many phases, without online measurements of the particle size. (C) 2003 Wiley Periodicals, Inc.