The structure of a fully deuterated sample of the conducting polymer poly(3-n.decylpyrrole) was investigated by neutron and x-ray diffraction. The different sensitivity of the two probes to deuterium atoms allowed us to obtain novel insights about the intermolecular organization of the polymer, with special focus on the alkyl substituent. The polymer turns out to be arranged in parallel layers. Within each layer, neighboring pyrrole rings maintain a coplanar disposition, whereas the alkyl decane tails are kinked out of the pyrrole plane. Monomers belonging to different adjacent layers display a high degree of superposition, due to a considerable overlap of both pyrrole rings and decane tails. Thus, despite its folded conformation, the alkyl substituent does not affect the short-range order of the compound, but rather contributes to determine its layered structure. (C) 2003 American Institute of Physics.