Self-assembled monolayers (SAMS) of octanethiol and benzeneethanethiol were deposited on clean Pt(111) surfaces in ultrahigh vacuum (UHV). Highly resolved images of these SAMS produced by an in situ scanning tunneling microscope (STM) showed that both systems organize into a super-structure mosaic of domains of locally ordered, closely packed molecules. Analysis of the STM images indicated a (root 3 x root 3)R30 degrees unit cell for the octanethiol SAMS and a 4(root 3 x root 3)R30 degrees periodicity based on 2 x 2 basic molecular packing for the benzeneethanethiol SAMS under the coverage conditions investigated. SAMS on Pt(111) exhibited differences in molecular packing and a lower density of disordered regions than SAMS on Au(111). Electron transport measurements were performed using scanning tunneling spectroscopy. Benzeneethanethiol/Pt(111) junctions exhibited a higher conductance than octanethiol/Pt(111) junctions.