In plasma etching processes, the reactor wall conditions can change over time due to a number of intentional and unintentional reasons, leading to a variability in the radical number densities in the plasma, caused by changes in the probabilities for reactions such as recombination at the walls. This leads to loss of reproducibility in the etching process. Here the authors isolated one such effect in which the feed gas was changed in the absence of a substrate. The transient surface composition of an anodized aluminum surface was determined for inductively coupled plasmas as the gas was switched from Cl-2 to O-2 and vice versa. The study was carried out with the spinning wall method and Auger electron spectroscopy. When the surface was first conditioned in an O-2 plasma and then exposed to Cl-2 plasmas, a rapid uptake of Cl was found in the first tens of seconds, followed by a slow approach to a steady-state value within similar to 5 min of plasma exposure. Conversely, when the surface was exposed to a Cl-2 plasma for a long time and then switched to an O-2 plasma, the anodized aluminum surface underwent a rapid dechlorination in the first few seconds and then a slow approach to steady state over similar to 3 min. Throughout these treatments, the coverages of Si (from erosion of the quartz discharge tube) and O were nearly constant.