Measurements with a low-pressure impactor and a differential mobility analyser were conducted in order to determine the fractal-like dimension (d(qf)) of diesel soot particles. Particles with diameters ranging from 55 up to 260 nm, were investigated at various engine loads. The resulting fractal-like dimensions ranged from 2.1 to 2.9 depending on the engine load. Similar measurements were done with a spark-ignition engine and yielded values of 2.2 or 3.0 depending on how the particles were conditioned.