The structure of aldehyde-terminated alkanethiol self-assembled monolayers (SAMs) on Au(111) is investigated using scanning tunneling microscopy (STM), atomic force microscopy (AFM), and density functional theory (DFT) calculations. For the first time, the structures of aldehyde-terminated SAMs are revealed with molecular resolution. SAMs of 11-mercapto-1-undecanal exhibit the basic (,root 3 x root 3)R30 degrees periodicity and form various c(4 x 2) superstructures upon annealing. In conjunction with DFT studies, the models of the (root 3 x root 3)R30 degrees and the c(4x2) superstructures are constructed. In comparison with alkanethiol SAMs, the introduction of aldehyde-termini results in smaller domain size, lower degree of long-range order, large coverage of disordered areas, and higher density of missing molecules.and other point defects within domains of closely packed molecules. The origin of these structural differences is mainly attributed to the strong dipole-dipole interactions among the aldehyde termini