Mandelic acid has cosmetic, pharmaceutical, and antibacterial activities and is used in urinary antiseptic medicines. An attractive process for the production of mandelic acid is through reaction between benzaldehyde, sodium hydroxide, and chloroform in the presence of polyethylene glycol 4000 as a phase transfer catalyst. The liquidliquid phase transfer catalyzed (LL PTC) reaction can be intensified by converting it into three-liquid phases (LLL PTC). We address the modeling of a well-stirred reactor for the foregoing process, in which organic droplets surrounded by a thin film of catalyst-rich phase are suspended in the aqueous phase. A population balance model is formulated for the LLL PTC reaction and solved by Monte Carlo simulation using interval of quiescence technique. Transport processes and intrinsic reaction kinetics are extracted from the experiments. This population balance model serves to assess and interpret the relative roles of various processes in LLL PTC reaction, such as diffusive transport, reaction, and interaction between dispersed phase droplets. The model is expected to be an effective tool for reactor design and scale up. (C) 2012 American Institute of Chemical Engineers AIChE J, 2012