The structure of aldehyde – terminated self-assembled monolayers (SAMs)/Au(111) is revealed for the first time by high-resolution scanning tunneling microscopy (STM) and atomic force microscopy (AFM). SAMs of 11-mercapto-1-undecanal (HS(CH2)10CHO) were prepared by solution immersion before being transferred to a UHV-STM chamber. Small domains displaying the basic (root3 x root3)R30° surface order was observed for as-deposited SAMs. Upon UHV annealing at 325 – 365 K, the size of ordered domains increased in conjunction with the appearance of c(4x2) superstructures. In comparison to alkanethiol SAMs, the introduction of aldehyde-termini results in a lower degree of long-range order and a higher density of point defects, from both STM and AFM investigations. The poster will also discuss the surface structure of aldehyde-terminated SAMs at saturation and low coverage. The understanding of the structure of pure and mixed aldehyde – alkanethiol SAMs is important for their application in protein immobilization and biochemical reactions.