A series of alpha'-mercaptomethylthiophenes with substituents at the alpha position were synthesized. The STM images of the SAMs of the thiophene derivatives showed domain structures with constant row-to-row spacings of 14 and 15 Angstrom for methyl and mesityl derivatives, respectively. However, the anthracenyl derivative gave different structures. These results suggest that the substitution at the a position does not have strong effect on the row-to-row spacing unless the substituent is bulkier than a mesityl group.