Adsorption of cis- and trans-butene dicarboxylic acids commonly known as maleic acid and fumaric acid, respectively, on Pt(111) electrodes was investigated in HF solution using in situ scanning tunneling microscopy (STM). It was confirmed that both molecules are strongly adsorbed on the Pt(111) surface in the potential range of double-layer charging. High-resolution STM images revealed internal molecular structures of maleic acid and fumaric acid. A molecule of maleic acid had a triangular shape, while a zigzag shape was observed for fumaric acid. Although several packing arrangements were found to form in small domains, major domains had (root 13 x root 13)R14 degrees and (2 root3 x 2 root3)R30 degrees structures for maleic acid and fumaric acid, respectively. These differences were explained in terms of the difference in conformation between the cis- and trans-isomers.