In the present work, the Aerosizer((R)) with the Aerodisperser((R)) is used to assess the size and size distribution of two common dry powder inhalers: Ventilan Rotacaps(TM) (salbutamol sulfate) and Bricanyl Turbohaler(TM) (terbutaline sulfate), according to a mixed factorial design. The different parameters of the equipment (voltage applied to the detector, shear force, deagglomeration, feed rate and pin vibration) were the factors considered in the design, set to different levels. Altogether, a full 4(1) x 3(1) x 2(3) factorial was considered and the results of the experiments analyzed according to the Montgomery technique. Results have shown that samples with a single population of particles were less affected by the changes in the parameters, whereas for mixtures incorporating different populations results were affected upon change of the levels of the factors. The voltage at the detector was the most important factor, followed by the shear force. By contrast, pin vibration and feed rate were less significant. The interactions between variables were also responsible for significant changes in the results and should not be neglected throughout the measurements. (C) 2003 Elsevier Ltd. All rights reserved.