Mass transfer of chlorobenzene in sulfuric acid is very important in the nitration of chlorobenzene by mixed acid. This article presents the determination of mass transfer coefficient at high-concentrations of sulfuric acid at 298 K and 313 K at 2 pi rad/s and 4 pi rad/s impeller speeds. The experiments were carried out under flat-interface conditions in an unbaffled glass reactor of 10 cm diameter and 1 dm(3) capacity. A glass stirrer with two paddle-type impellers was employed for stirring. The upper paddle stirred the organic phase and the lower paddle stirred the aqueous phase. High mass transfer coefficients were obtained at high-concentrations of sulfuric acid. Dynamic light scattering of the aqueous phase revealed that a dispersion of chlorobenzene spontaneously formed in sulfuric acid, which is believed to be the main factor behind the high mass transfer coefficient. NMR analysis of the aqueous phase showed that no chemical reaction occurred at 298 K and 313 K, but some amount of 4-chlorobenzenesulfonic acid formed at 323 K. The high mass transfer coefficient is expected to result in high yield of chloronitrobenzenes at moderate temperatures and low stirring speeds. (C) 2011 Elsevier Ltd. All rights reserved.