Nanowires of zinc blende crystal structure, grown in the < 111 > B direction usually have a large number of twin plane defects. In order to investigate this phenomenon, we grow GaP nanowires with metal-organic vapor phase epitaxy. By rotating the nanowires in a high resolution transmission electron microscope, we show that the nanowire segments between the twin planes are of octahedral shape and are terminated by {111} facets. Due to the alternating orientations of these twin octahedra, the sidewalls of the nanowires can be described as microfaceted surfaces with an overall orientation of 11 1 2}, but composed of alternating 11 1 I}A and {1 I 1}B facets. Moreover, the segment thicknesses follow exponential distributions, which show that there is a certain probability of twin plane formation, which is independent of segment thickness. (c) 2006 Elsevier B.V. All rights reserved.