The effects of surface contamination of the surface of Langmuir probes have been studied experimentally. These experiments were carried out in three different argon glow discharge tubes. One tube was designed to simulate conditions found in large plasma facilities used in engineering studies, while the other two were designed and prepared in accordance with high‐purity laboratory standards. In addition, the effect of a small amount of air on an argon plasma was investigated. The data are presented in the form of original probe traces taken on a two‐axis recorder. These data were used to construct curves of contamination versus time and of the logarithm of electron current versus probe voltage. It was found that sufficient contamination is present, even in relatively well‐prepared systems, to cause gross errors in probe measurements. The errors are found to be especially insidious in that the self‐consistency checks commonly used in Langmuir probe applications do not reveal the presence of contamination. It is concluded that probe measurements will be compromised unless particular attention is given to the problem of maintaining a clean probe.