A fast integrated mobility spectrometer (FIMS) has been developed for rapid aerosol size distribution measurements including those aerosols with low particle number concentrations. In this work, an inversion routine has been developed for the FINIS and it is demonstrated that the FIMS can accurately measure aerosol size distributions. The inversion routine includes corrections for the particle residence time in the FINIS and other factors related to the width of the response (or transfer) function and multiple charging of particles. Steady-state size distributions measured with the FIMS compared well with those measured by a scanning mobility particle sizer (SNIPS). Experiments also show that the FIMS is able to capture the size distribution of rapidly changing aerosol populations. The total particle concentration integrated from distributions measured by the FIMS agrees well with simultaneous measurements by a condensation particle counter (CPC). (C) 2008 Elsevier Ltd. All rights reserved.