Pore propagation during anodization of (100) n-InP electrodes in aqueous KOH was studied in detail by scanning and transmission electron microscopy (SEM and TEM). Pores emanating from surface pits propagate along the 111A crystallographic directions to form, in the early stages of anodization, porous domains with the shape of a tetrahedron truncated symmetrically through its center by a plane parallel to the surface of the electrode. This was confirmed by comparing the predictions of a detailed model of pore propagation with SEM and TEM observations. The model showed in detail how 111A pore propagation leads to domains with the shape of a tetrahedron truncated by a (100) plane. Observed cross sections corresponded in detail and with good precision to those predicted by the model. SEM and TEM showed that cross sections were trapezoidal and triangular, respectively, in the two cleavage planes of the wafer, and TEM showed that they were rectangular parallel to the surface plane, as predicted. Aspect ratios and angles calculated from observed cross sections were in good agreement with predicted values. The pore patterns observed were also in good agreement with those predicted and SEM observations of the surface further confirmed details of the model. © 2013 The Electrochemical Society. [DOI: 10.1149/2.107306jes] All rights reserved.