Scanning tunneling microscopy with very high gap impedance reveals several stable molecular conformations of organic monolayers self-assembled from alkanethiols on gold. Thermal annealing of the preassembled films favored well-packed large molecular domains that were studied in real space with unprecedented detail and accuracy. At room temperature the system has several stable molecular conformations, which implies that chemisorption energies balance the energy of interaction between molecular chains. A similar probability of different conformations indicates almost equal total energies, which makes it an interesting-and challenging-object for molecular dynamics simulations. Our results contribute to a long-standing scientific debate on the structure of self-assembled monolayers and provide the basis for an improved understanding of organic interfaces.