A dynamic phenomenon that occurs with the measurement of a diluted diesel-soot aerosol with an electrical low-pressure impactor (ELPI) was studied. An experimental setup for measuring the performance of diesel particulate filters was used for this purpose. Impactor overloading is thought to be responsible for this dynamic phenomenon. SEM analysis indicates that the fluffy or fractal structure of the diesel-soot aerosol is responsible for rapid overloading. A fluffy bed of particles on the impactor surface filters the particle-laden gas stream as it passes over the impactor surface. This phenomenon could lead to improper use of the ELPI and thereby it poses a restriction for this application. Users in the field should be aware of this phenomenon because the transient response signal from the ELPI could easily be misinterpreted as a start-up effect whereas, in reality, the ELPI is used improperly. The use of oil-soaked sintered collectors can be a solution for this problem.