In the current work a detailed study is performed of the turbulent-to-laminar transition at the inlet of an automotive monolithic reactor. The effects of the transition on the conversion of gaseous species and on the deposition of particulate matter are investigated using numerical simulations. Two main effects of the turbulence on the conversion in the monolith have been identified: slow fluctuations due to turbulent eddies that are too large to enter the channels and rapid fluctuations due to smaller turbulent eddies that penetrate the channels. It is also shown that inertial particles may deposit inside the monolith channels, providing a likely explanation of the experimentally observed spatial deposition profiles of elements that chemically deactivate automotive catalysts.