Anodic Aluminum Oxide (AAO) has a variety of applications depending on its pore diameter, uniformity and porosity. Here, we investigate a new finding of the widened pore diameter of AAO greatly linked to the oxalic acid anions contaminated in the AAO. As AAO was prepared by hybrid-pulse anodization in oxalic acid at low-to-high concentrations (0.3 similar to 0.9 M) and room temperature (25 degrees C) instead of conventional low temperature (0 similar to 10 degrees C) and low concentration (0.3 M), the average pore diameter roughly increased with concentration from 42.8 nm to 46.5 nm; while it inversely decreased with concentration from 85.6 nm to 80.0 nm after widening to enhance all pore diameters. High oxalic acid concentration resulted in AAO walls containing more anions that further decreased etching rate during widening. This new phenomenon became more pronounced by direct-current anodization than by the hybrid-pulse anodization. The possible mechanism of evolution of AAO pore diameter during pore widening in the phosphoric acid was proposed and supported by the plane-view and cross-section SEM results and photoluminescence spectra. (C) 2017 The Electrochemical Society. All rights reserved.