Conformational polymorphism of an oligothiophene, the title compound (T4S), is reported for the first time. Efficient synthesis of T4S produced, in addition to the triclinic, deep orange crystals reported previously, a second (yellow monoclinic) form of T4S. whose structure as revealed by single-crystal X-ray diffraction is presented here. The synthetic pathway is outlined and the molecular structure and crystal packing are dis cussed. The significantly different pi-delocalization in the two conformations indicates the need for very careful molecular characterization.