The fast integrated mobility spectrometer (FIMS) is a highly sensitive instrument with a fast response time. The time response of the FIMS is limited by the mixing of the aerosol in the inlet of the instrument, which “smears” the detected aerosol over a range of time. The response time is also limited by the different transit times that particles experience in the instrument due to differences in particle classification trajectories. Experiments show that the difference in particle transit times can be corrected using a simple model of the particle trajectories. Furthermore, the “smearing” effects in the inlet can be corrected by de-convolving the time series of particle counts (the temporal de-convolution) in each size bin before inverting the data. The dynamic response of the FIMS was investigated by measuring an aerosol subjected to a step-change and sinusoidal-change in number concentration. The attenuation of the FIMS signal, without using the temporal de-convolution, was measured with and without an aerosol neutralizer in the FIMS inlet. Due to its large internal volume, the neutralizer significantly slows down the time response of the FIMS when installed in the inlet. For a sinusoidal signal at 0.33 Hz, the measured attenuation of the FIMS without the neutralizer was 56% versus an attenuation of 82% when the neutralizer was used. When the temporal de-convolution was applied at the same frequency, the FIMS was able to capture the full variation of the aerosol size distribution; however the random noise in the derived size distribution was amplified.