Sub-micrometer layers of electrochemically prepared methyl- and decyl-substituted poly(3,4-ethylenedioxythiophene) (PEDOT) carrying perchlorate counterions have been examined with grazing incidence X-ray diffraction with synchrotron radiation. The materials were found to be partially crystalline, and the data could be ascribed to a model of sheets of pi-pi stacked polymer chains with a smectic ordering of these sheets. An unsubstituted PEDOT sample with the polymeric polystyrenesulfonic acid as a counterion was also investigated and turned out to be essentially amorphous. (C) 2003 Wiley Periodicals, Inc.