Self-assembled aggregates of amphiphilic surfactant molecules formed on solid surfaces are similar to biological membranes. To understand the formation mechanism of these aggregates, we have studied the formation of self-organized monolayers from low-concentration sodium dodecyl sulfate (SDS) aqueous solutions (concentration below the critical micelle concentration) on gold surfaces. The study has been carried out by using simultaneously quartz crystal microbalance (QCM) and open circuit potential measurements in situ. We have developed a model which explains the variation of the QCM frequency and open circuit potential following SDS additions to water. The dominant growth mechanism during the major part of film formation was demonstrated to be surface diffusion of surfactant molecules.