The most commonly used quantity to characterize surface cleanliness through X-ray photoemission spectroscopy (XPS) measurements is the so-called relative atomic surface concentration of carbon (at.% C). We have investigated the relationship between at.% C values and the C Is peak area on Cut and we find a nearly linear behaviour in the range 15-80 at.% C. Correction factors for the measured at.% C values that enable a comparison of the cleanliness level of different materials, notably Cu, Al and stainless steel, have been determined experimentally. The influence of the storage time and method on the degree of re-contamination of initially clean Cu has been examined. The carbon contamination on clean metallic Cu increases abruptly to some 20 at.% C upon air exposure and continues to increase with storage time in air. Storage in polymer bags can lead to up to 70 at.% C after 1 month, whereas storage in aluminium foil can preserve an acceptable surface cleanliness for a similar storage time. (c) 2005 Elsevier B.V. All rights reserved.