We have studied the dependence of the surface roughness of electrochemically anodized p-type silicon on ion irradiation fluence. For moderate resistivity wafers, the surface roughness reduces with fluence, consistent with the dominant effect being a reduced anodization rate. However, for low resistivity wafers, the surface roughness increases with fluence. This is explained by showing how irradiation converts the low wafer resistivity, which tends to form mesoporous silicon with low associated roughness, into a moderate resistivity, which tends to form microporous silicon with high associated roughness. This result explains why the anomalous behavior of surface roughness and photoluminescence intensity is observed. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3481769] All rights reserved.