Starting from two-step anodizing recipes available in the literature, we fabricated selfsupporting ordered ion-implantation masks that are several mm2 in area and 1-4 νm thick. SEM micrographs reveal self-organized structures with straight open pores, 25-150 nm in diameter, extending completely through the mask. As reported previously, the pore diameter and spacing depend critically upon the anodization parameters, e.g., type of acid and its molality, the applied voltage and the solution temperature. Ion-milling procedures were developed for opening the bottoms of the anodized pores. These masks appear quite robust during exposure to ion beams of 1-MeV He, Ne, and Kr. The steps necessary to fabricate the implantation masks, including opening the pores, are briefly described. Here we present new results obtained with a mask fabricated with pore dimensions as small as 25 nm in diameter, i.e., at the limit of what is technically feasible. Measurements are reported of the angular dependence of the transmitted ion current; these results are consistent with the physical dimensions of the opened pores. TEM images of a partial array obtained by implantation through the 25-nm pores are also shown.