Open filters (with low pressure drop) have potential for energy-efficient reduction of particulate matter (PM) from engines. In the work reported here, the capture efficiency of PM in open substrates has been investigated using PM from a real engine under various flow conditions and sampling settings. The observed capture efficiency (CE) confirmed the expected trends that increased residence time and increased temperature give better CE. However, the volatile content (assumed to be hydrocarbons, HC) can increase the apparent CE due to rapid evaporation and/or shrinkage of the PM. In order to quantify these effects, a conceptual model has been implemented that can be used as an in situ analyzer of the PM properties. The results show how exhaust treatment (heating and/or dilution) changes the characteristics of the PM. These properties affect CE and can be used for subsequent catalyst optimization. In addition, the method developed here was used to analyze nucleation-mode PM from a special fuel injection strategy. The results revealed that these particles were mainly nonvolatiles, demonstrating the usefulness of this characterization methodology. Furthermore, an equation for diffusion losses in the rotary dilutor for the DMS500 is presented.