A water-cooled fluidized bed particle trap has been built and tested far diesel particle agglomeration. A theoretical formula incorporating thermophoretic mechanisms into normal fluidized bed filtration has been derived to calculate particle removal efficiency in a water-cooled fluidized bed and validated with published data and measurements from this study. Reasonably good agreement between the measurements and model predictions is achieved. A test of effectiveness using a modern heavy-duty diesel engine showed encouraging results. A significant reduction of total sulphur in the gas phase was also found. particle number concentrations in different size ranges were monitored in real time with an ELPI as engine load was changed, and showed a sharp transient increase in particle number after the trap as engine load was increased The carbon volatility spectra of particles collected up- and downstream of the fluidized bed were compared, and revealed a change in composition occurring within the trap. Water-tooled fluidized bed filtration can efficiently reduce existing submicron particle emissions and prevent potential ultrafine particle formation in the exhaust gases.