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 1s peak area on Cu and we find a nearly linear behaviour in the range 15–80at.%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 20at.%C upon air exposure and continues to increase with storage time in air. Storage in polymer bags can lead to up to 70at.%C after 1 month, whereas storage in aluminium foil can preserve an acceptable surface cleanliness for a similar storage time.