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Magnetic resonance imaging (MRI) has been used to investigate gas flow in diesel and gasoline particulate filters, exploring gas flow both within the channels and also at the entrance and exit of the filters (expansion and contraction effects). The latter measurement can be used to measure turbulent diffusivity in the filter bulk flow characteristics, and therefore estimate the importance of entrance and exit effects in contributing to overall filter back pressure as the filter properties and/or exhaust flow changes (i.e. with Reynolds number). The former measurement gives information that can be used to evaluate filter performance, in particular with respect to filtration efficiency, and examples will be shown from our measurements on diesel filter systems.