Using scanning tunnelling microscopy (STM), we have studied mixed self-assembled monolayers of linear alkanethiol molecules. Nonanedithiol (C9S2), nonanethiol (C9S), decanethiol (C10S), and dodecanethiol (C12S) were inserted into a self-assembled octanethiol (C8S) host matrix monolayer on an Au(111) surface using a two-step method. Quasi-one-dimensional double-row structures were found in the ordered, close-packed domains of the C8S matrix for each mixed monolayer system. These close-packed domains coexist with ordered striped phase domains (for C9S and C10S) or with a disordered phase (for C9S2 and C 12S). Results from high-resolution images suggest that the double-rows are composed of inserted non-nearest-neighbor substitutional molecules, the ordering of which may be a result of locally induced surface stress.