In this study of the transfer function of copies of the Vienna-type differential mobility analyser (DMA) it was found that three, supposedly identical, DMAs exhibited distinct differences with respect to the width of the transfer function. This means that the transfer function of a DMA needs to be determined on an individual basis in order to obtain accurate results from DMA measurements. The size dependence of the transfer function width and the penetration of three DMAs were investigated experimentally in a broad range of Peclet numbers (400-400000). Two of the three DMAs exhibited transfer function widths in excess of the broadening induced by diffusion. The size dependence of the width of all three DMAs could be explained within a few percents by assuming an instrument-specific, size-independent broadening in addition to diffusion broadening. The penetration was evaluated by relating to deposition in a circular tube. Excellent agreement between the experimental data and the simple model was obtained. The penetration study was extended to include literature data, showing that the simple model could be applied and that there are distinct differences between different DMA types. The results of this study indicate that the size dependence of the width of the transfer function and the penetration can be estimated by measuring the transfer function for a single particle size, thus greatly reducing the amount of work required by DMA users to characterise their instruments. (C) 2003 Elsevier Science Ltd. All rights reserved.