A methodology for studying the deagglomeration performance and emptying behaviour of micronised mannitol powder from two commercial capsule-based dry powder inhalers (DPIs), the low (LR) and high resistance (HR) RS01®, is presented. Mathematical modeling played a key role in the interpretation of the powder release behaviour from these two DPI systems. Non-linear regression models were used to estimate the rate constants for emptying of mannitol powder, which were characterized from the aerosol obscuration versus time profiles obtained from laser diffraction particle sizing data. The effects of device resistance and associated pressure drops, sampling flow rate, rates of powder emptying and the presence of a capsule on the dispersion characteristics were studied. The presence of a capsule significantly improved the aerosolisation performance of mannitol powder from both inhalers, which may be due to the extended powder-air-device interactions within the device. It is important to consider the stochastic nature of movement and physical state of the capsule when assessing the aerosolisation mechanisms and dispersion performance from these complex delivery systems. The methodology set out in this study has the capacity to provide a greater level of detail in the study of aerosol plume characteristics from capsule-based DPIs.