The effect of porosity and surface roughness of collection substrates have been studied in a cascade multijet low-pressure impactor. Studied substrates included porous substrates with pore sizes of 0.5, 5 and 40 mum and solid substrates with sandblasted surface. Higher pore sizes and increased roughness lead to smaller cut-point and less steep collection efficiency curve. For smaller pore sizes the increase in the collection can be explained with the surface roughness of the substrate. The effect of fluid penetration was significant only for the 40 mum pore size. Changes in the collection efficiency are so large that impactor needs to be recalibrated if the substrates are changed. (C) 2003 Elsevier Ltd. All rights reserved.