An experimental study of mean drop size (d(32)) has been conducted in a horizontal mixer-settler for toluene/water system. The effects of feed holdup and agitation speed on d(32) were investigated in the absence and presence of silica nanoparticles. In order to study the influence of silica nanoparticles, different amounts of them (0.005, 0.01, 0.03 and 0.05 wt.%) were dispersed in water using ultrasonic vibration. The stability of nanofluids was evaluated by means of an UV-vis spectrophotometer. It was found that introducing silica nanoparticles in the system can decrease d(32) up to similar to 21% in different operating conditions. Furthermore, presence of silica nanoparticles lowers the increasing influence of feed holdup and decreasing influence of agitation speed on d(32). Initially, an empirical correlation for predicting d(32) was proposed from experimental results of pure system. After that, new general correlations were proposed for systems containing different concentrations of silica nanoparticles which included the term of weight fraction of silica nanoparticles being in good agreement with experimental data. (C) 2013 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.