The structure of self-assembled monolayers of 4-mercaptopyridine on Au(lll) has been investigated in perchloric acid solutions by in situ scanning tunneling microscopy (STM), The adsorption of this molecule yields an ordered striped structure with a rectangular (5 x root 3) unit cell in the double-layer potential range. The sulfur headgroup and pyridine ring moiety of the molecule were clearly imaged. The molecular image is greatly different from that reported previously. The spacing between sulfur headgroups of adjacent molecules was found to be 0.21 +/- 0.02 nm. This result strongly suggests that the molecules are dimerized at the sulfur position to form a disulfide. The disulfide has a trans-CSSC skeleton, and both the pyridine rings are oriented nearly parallel to the surface. This open structure enables the direct STM observation of the sulfur-sulfur bond.