The alpha-conjugated and structurally defined dodecithiophene 1 well crystallizes from solution into single crystals which were investigated by X-ray structure analysis. Molecular conformations and packing of the longest oligothiophene ever structurally characterized were determined. Surprisingly, rather rare syn-conformations at the terminal thiophene rings and partial twisting of alkyl side chains for compact packing were found, whereby the molecules form a layer-like arrangement with partial overlap of their heteroaromatic systems. In comparison, the molecular properties of the oligomer in a 2D self-assembled monolayer adsorbed at the liquid-solid interface were investigated by scanning tunnelling microscopy (STM). In comparison to the 3D bulk material, on graphite, a different molecular arrangement was found. A lamella-type ordering of molecularly resolved oligomers which exhibit an all-anti conformation could be imaged over wide areas. Larger lattice constants were determined which originate from molecule-substrate interactions.