The coupling between thermal mass and buoyancy-driven natural ventilation has great potential for passive regulation of indoor thermal environment. The nonlinearity associated with the use of this coupled mode has attracted great attention. Theoretical models have been developed for decoupling indoor air temperature and ventilation flow rate for buildings exposed to harmonically fluctuating external thermal environments. However, the solutions in existing models are implicit and only the main fluctuation frequency of indoor environmental parameters is accounted for. This paper focuses on multi-frequency fluctuation behaviors induced by the coupling between thermal mass and buoyancy-driven natural ventilation. A mathematical approach is undertaken to decouple the indoor air temperature and ventilation flow rate, and explicit solutions are obtained for both the phase shift and fluctuation amplitude of the indoor air temperature and ventilation flow rate at various frequencies. Experiments are performed to validate the theoretical analysis. Both analytical and experimental results show that coupling between thermal mass and buoyancy inside a building can lead to multi-frequency (or anharmonic) fluctuation of natural ventilation. The phase shifts of the high-order indoor air temperature fluctuation terms (with respect to the 1st-order outdoor air temperature fluctuation term) can exceed pi/2. (C) 2016 Elsevier Ltd. All rights reserved.