Self-assembled monolayers (SAMs) composed of 15-hydroxypentadecanoic acid and 16-hydroxyhexadecanoic acid are investigated with scanning tunneling microscopy (STM) at the solution-graphite interface. The observed packing structures of these two molecules on the surface are dramatically different and reveal different intermolecular bonding arrangements exhibiting a new type of "odd/even" chain-length effect. In addition, solvent coadsorption has been found to occur in the case of 15-hydroxypentadecanoic acid, and an unusual surface pattern in which the molecular axes in neighboring lamellae are completely aligned with one another is observed for 16-hydroxyhexadecanoic acid.