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Controlling the self-assembly of thiophene-containing molecules and polymers requires a strong fundamental understanding of the relationship between molecular features and structure-directing forces. Here, the effects of ring-substitution position on the two-dimensional self-assembly of monosubstituted thiophenes at the phenyloctane/HOPG interface are studied using scanning tunneling microscopy (STM). The influence of π···π-stacking, hydrogen-bonding, and alkyl-chain interactions are explored computationally. Alteration of the amide attachment point from the 2- to the 3-position induces transformation from head-to-tail packing to head-to-head packing. This may be attributed to canceling of lateral dipoles.