The shape of the spectral features in arrival time distributions (ATDs) recorded by ion mobility spectrometry (IMS) can often be interpreted in terms of the coexistence of different isomeric species. Interconversion between such species is also acknowledged to influence the shape of the ATD, even if no general quantitative description of this effect is available. We present an analytical model that allows simulating ATDs resulting from interconverting species. This model is used to reproduce experimental data obtained on a bistable system and to interpret discrepancies between measurements on different types of instruments. We show that the proposed model can be further exploited to extract kinetic and thermodynamic data from tandem-IMS measurements.