We report on the use of in-situ optical emission spectrometry to monitor the Al3+ loss rate during the anodic oxidation of aluminum in phosphoric acid. Three distinct stages were observed, the highest rate being measured during barrier layer growth. The evolution of the loss rate is markedly different from the evolution previously reported for sulfuric acid anodizing. We speculate that this may be related to the different pore morphologies obtained in these electrolytes. Our measurements also indicate that below 2 mA/cm(2), field-assisted dissolution is the predominant contribution to Al3+ loss, while direct cation ejection is predominant at higher current densities. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.009201esl] All rights reserved.