The rate of nitration of chlorobenzene and toluene by mixed acid has been studied in a small batch reactor. The rate of nitration of chlorobenzene was studied in both homogeneous and two-phase systems. Measurement of the solubility of chlorobenzene in aqueous sulphuric acid enabled an excellent correlation to be obtained between the single- and two-phase rate data. In this way kinetic control of the two-phase reaction rate was demonstrated. An identical system was used to study the rate of nitration of toluene in a chlorobenzene solution. The experimental results showed that under these conditions, the mass transfer and kinetic rates were competing for control of the overall reaction rate. A novel method of plotting the experimental data enabled values of the volumetric mass transfer coefficient and the homogeneous second order rate constant to be obtained.