In this work the occurrence of two polytypic structures, chalcopyrite (CH) and cationic fcc CuAu-ordering (CA), in epitaxial CuInS2 samples on Si (1 1 1) oriented substrates has been studied. Samples with different Cu-to-In ratios and grown at 500 and 575 degreesC, respectively, have been analysed. Cross-section Transmission electron microscopy (TEM) samples have been prepared and studied in two directions normal to the growth direction [2 2 1](CH), namely, [1-1 0](CH), which is parallel to [1 0 0](CA), and [1 1 1](CH) that is parallel to [0 1-1](CA). Electron diffraction and high resolution TEM images corroborate the existence of both polytypes and give the orientation relationship between them, namely, [1-1 0](CH)parallel to[1 0 0](CA) and (1 1 2)(CH)parallel to(0 1 1)(CA). Cu-rich samples have good crystalline quality with large grains and sharp interfaces between both polytypes, the most important being between the (1 1 2)(CH) and (0 1 1)(CA) planes. On the contrary, the crystalline quality of Cu-poor samples is minor, no clear interfaces can be observed, and a high density of twins in the (0 I-I)c, planes can be observed. The results on the crystalline quality are in agreement with previous results on polycrystalline CuInS2 films. (C) 2003 Elsevier Science B.V. All rights reserved.