The effect of scale, processing conditions, interfacial tension and viscosity of the dispersed phase on power draw and drop size distributions in three in-line Silverson rotor-stator mixers was investigated with the aim to determine the most appropriate scaling up parameter. The largest mixer was a factory scale device, whilst the smallest was a laboratory scale mixer. All the mixers were geometrically similar and were fitted with double rotors and standard double emulsor stators. 1 wt.% silicone oils with viscosities of 9.4mPa s and 339 mPa s in aqueous solutions of surfactant or ethanol were emulsified in single and multiple pass modes. The effect of rotor speed, flow rate, dispersed phase viscosity, interfacial tension and scale on drop size distributions was investigated. It was found that for all three scales, power draw is the sum of the rotor and flow contributions, with proportionality constants, Po-z and k(1), that are practically scale independent. Sauter mean drop size appeared to correlate better with tip speed than energy dissipation rate. For ethanol/water solutions, mean drop size correlated well with Weber number based on interfacial tension, but for surfactant solutions effective interfacial tension gave better correlations with Weber number. (C) 2013 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.