Volatile particles cannot be detected at the engine exhaust by an aerosol detector, as they are formed when the exhaust is mixed downstream with the ambient air. The lack of a precise definition of volatile engine particles has been an impediment to engine manufacturers and regulatory agencies involved in the development of effective control strategies. Volatile particles from combustion sources contribute to the atmospheric particulate burden, and this is a critical issue in ongoing research in the areas of air quality and climate change. A new instrument, called a volatile particle separator (VPS), is developed in this work. It utilizes a proprietary microporous metallic membrane to separate particles from vapors. VPS data are used in the development of a two-parameter function to quantitatively classify, for the first time, the volatilization behavior of engine particles. The value of parameter “A” describes the volatilization potential of an aerosol. A nonvolatile particle has a larger A-value than a volatile one. The value of parameter “k,” an effective evaporation energy barrier, is found to be much smaller for small engine particles than for large engine ones. The VPS instrument is not simply a volatile particle remover, as it makes possible the characterization of volatile engine particles in numerical terms.