The effect of the residual swirl from the turbocharger turbine on the catalyst flow distribution has been investigated experimentally and numerically. A swirling flow rig with a moving-block swirl generator was used to generate swirling flow in a sudden expansion diffuser with a wash-coated diesel oxidation catalyst (DOC) downstream. Hot-wire anemometry (HWA) was used to measure the axial and tangential velocities of the swirling flow upstream of the diffuser expansion and the axial velocity downstream the monolith. Pressure along the diffuser wall was measured using Scanivalve pressure scanners. With no swirl, the results show that the flow is highly non-uniform in the catalyst monolith with maximum velocities near the diffuser axis. High non-uniformity is also exhibited at high swirl levels with highest velocities near the diffuser wall. An intermediate swirl level exists where the flow is uniform. To gain further insight into the mechanisms controlling flow redistribution, numerical simulations have been performed using the commercial computational fluid dynamics (CFD) code STARCCM+. With no swirl, the central jet transverses the diffuser, and a drastic flow redistribution takes place near the monolith face due to its high resistance. The flow was also observed to separate from the walls downstream of the expansion. Increasing swirl reduces the size of these separation zones, and eventually leads to formation of the central recirculation zone characteristic to high swirl flows. At intermediate swirl levels, the wall separation zones are reduced and the axial adverse pressure gradient is insufficient to cause a central recirculation. This flow regime occurs at relatively low swirl levels (S ≈ 0.4) which may have positive implications for aftertreatment system design with low residual swirl levels from the turbine, which can also be tuned by adjusting the distance between the turbine and the catalyst or employing guide vanes.