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The aim of this paper is to characterize an electrostatic agglomerator with an innovative shape intended to collect diesel particles. The agglomerator consists of a tube-type electrofilter with a porous collecting electrode supplied by a negative dc operating voltage. The end of the device is closed, so all the treated gas passes through the collecting electrode. The stocking surface is very small, and the collecting electrode acts as a simple agglomerator. The specific electrical consumption is approximately 1 J/L for a 95% reduction in particle number. This agglomerator requires an extremely short residence time, in the same order of magnitude as the particle electrical charging time. The reentrained agglomerates clearly have micronic size and are very little in number, so that their characterization cannot be done easily by classical granulometers such as electrical low-pressure impactor or scanning mobility particle sizer. An approach to modeling the reentrained number distribution is proposed, which permits to confirm and explain many of the experimental results. Discrepancies between experimental results and numerical model predictions show that particle filtration may be highly enhanced by a dendrite layer actively maintained by an electric field.