An investigation of the permeability evolution of a diesel particulate filter channel wall as a function of soot loading was conducted. This investigation examined the effects of varying particle characteristics and two filtration velocities (4 and 8 cm/s) on the wall permeability throughout a 1 g/L soot loading. This study was possible using the Diesel Exhaust Filtration Analysis (DEFA) system that was modified to perform temperature controlled in-situ flow tests. The DEFA system allows for isolation of the pressure drop due to the filter wall and soot cake layer greatly simplifying the permeability calculation.Permeability evolution fundamentals and the effects of loading conditions were studied by filling 18 filters with the DEFA system. The filters were loaded using one of four operating conditions of a single-cylinder heavy-duty diesel engine. These operating conditions were comprehensively characterized giving insight into the effects of varying particle characteristics. Filter pressure drop and particle breakthrough were monitored during loading. Additionally, the loading process was ‘paused’ at several points to conduct in-situ flow tests resulting in a permeability evolution curve as a function of soot loading.It was found that both filtration velocity and particle characteristics have distinct influences on the filter wall permeability during deep bed filtration as a result of varying soot packing densities and depths of penetration. High filtration velocities and large concentrations of nucleation mode particles result in a lower permeability at the loaded wall condition. Since the soot cake has a relatively small contribution to overall permeability, the conditions that determine the loaded wall permeability essentially set the filter permeability for the majority of loading.