The results of an experimental study of the absorption properties of Cs as present in Cs‐Ar low‐pressure discharges under various discharge conditions are presented. The total Cs concentration at zero discharge current was varied between 1.5 and 15×1012cm−3. Absorption was measured in the 4555‐Å and 8521‐Å lines of Cs from which the stationary concentration of Cs (62S1/2) could be derived. Special attention was paid to determine the Cs concentration as the discharge changed from a low‐burning voltage mode to a high‐burning voltage mode. The pronounced variations of the discharge properties were found to be accompanied by rather drastic changes of the Cs concentration for which variations of a factor of ten or more were found.