The coadsorption of sodium dodecyl sulfate (SDS) and dodecanol in trace amounts from aqueous solutions is experimentally studied using dynamic surface tension (pi (t)), Brewster angle microscopy (BAM), and synchrotron X-ray diffraction at grazing incidence (GIXD). Comparative studies were made of the mixed systems vis a vis the pure components. The adsorbed monolayer mixed systems proved completely different in their dynamics and properties from those of the pure components. The adsorbed monolayers of the investigated dodecanol/SDS solutions undergo a first-order phase transition with subsequent formation of condensed phase domains. In adsorption equilibrium, the surface is largely covered by the condensed phase. The morphological features of the condensed phase resemble those of the pure dodecanol monolayer. Both the BAM studies and the GIXD data indicate hexagonal packing of nontilted molecules. The condensed phase formed by the dodecanol/SDS mixtures has exactly the same lattice structure as observed in pure dodecanol layers. Although present only as the minor component in the aqueous solution, at adsorption equilibrium, the fundamental features of dodecanol dominate largely the adsorbed monolayer of the mixed dodecanol/SDS solutions. A small extension of the lattice structure at coadsorption of the dodecanol/SDS mixtures is obviously caused by incorporation of a small proportion of SDS into the dodecanol lattice.