The reactive extraction of succinic acid with TOA dissolved in butyl acetate from viscous aqueous solutions which simulate the rheological behavior of Actinobacillus succinogenes broths underlined the strong influence of viscosity and pH on interfacial mass flow, mass transfer driving force, and mass transfer coefficient. The magnitudes of these effects are attenuated by 1-octanol addition into the solvent phase, the interfacial mass transfer rate being accelerated for 1.2-3 times, depending on the apparent viscosity and the pH-value of the aqueous solution. The influences of the extraction parameters and mechanism have been included in a mathematical model adequate for the solute mass transfer coefficient from aqueous to organic phase.