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The Scanning Mobility Particle Sizer (SMPS) and the Electrical Low-Pressure Impactor (ELPI) are frequently used to measure the number and size of combustion aerosols. The instruments are especially popular in the field of engine technology, where the emission of “particulate matter” is restricted by legislation. During the experiment with a “standard” ELPI it is observed that initially the number of small particles decreases rapidly, while simultaneously the number of larger particles increases. This non-ideal behavior of the ELPI can be overcome by the usage of oil-soaked sintered impactor stages. The interpretation of the results as measured by SMPS and ELPI is not straightforward due to the fractal character of the diesel soot agglomerates. The theory of fractal-like agglomerates is used to assess the performance of the ELPI and the SMPS for the measurement of diesel soot particles and is validated visually with electron microscopy (SEM).