We report the results of an ultrahigh vacuum scanning tunneling microscope (STM) study of n-alkanethiol [CH3(CH2)(n)-1SH] (C-n) monolayers self-assembled on Au(111) single-crystal surfaces. The STM images show that the saturation coverage self-assembled monolayer (SAM) unit cell measures 0.86 x 1.0 nm and contains four thiolate molecules. The observed unit cell is consistent with results from recent X-ray, electron, and helium diffraction studies. This unit cell corresponds to a c(4x2) superlattice of a basic (root 3x root 3)R30 dense-packed monolayer. The domain size ranges from 5.0 to 15 nm with domains separated by molecular-width rotational boundaries, antiphase boundaries, and stacking faults. SAMs composed of longer chain thiols (C-8 and C-10) were stable in vacuum for over 40 days.