The problem of sample cleaning is essential for all the scientists using ultra-high vacuum (UHV) techniques. The paper explores the issue of how the real structure of the monocrystalline sample affects its cleaning procedure. The mosaic structure of a monocrystalline sample should be taken into account in the interpretation of segregation and adsorption phenomena studies carried out under UHV conditions. Some examples of the cleaning of an Fe(1 1 1) sample from sulphur and carbon impurities are presented in the paper. Cleaning may result in obtaining two different states: clean surface and clean defects or clean surface but filled defects. According to these studies, the number of adsorption sites in the defects equals approximately 50% of the number of adsorption sites on the surface. (c) 2005 Elsevier B.V All rights reserved.