Adsorption of surfactant molecules from bulk solution to air/water interface is important in various applications such as cleaning products, biological membranes and so on. For understanding the nature of adsorbed surfactant layer it is important to measure the surface density of adsorbed layer and also characterize its structure. External reflection-absorption Fourier transform infrared spectroscopy (ERA FT-IR) is a powerful technique for characterizing the structure and composition of adsorbed layer at air/water interface. In the present work the surface density and structure of adsorbed layer of HTAB, DTAB and C12E1 were investigated at different bulk concentrations and constant temperature by ERA FT-IR method. In all systems the adsorption isotherms (surface concentration vs molality) at relatively low concentrations show a Frumkin type pattern. For DTAB around cmc and C12E1 at high bulk concentrations the surface density increases again. This increase in surface densities will be discussed in detail. The surface density values which obtained by ERA FT-IR technique were compared with results of other techniques such as surface tension and XAFS measurements. The structures of adsorbed molecules at different concentrations were determined by wavenumber values at peak maximum. The results show that there is a surface phase transition from gaseous to liquid expanded state for HTAB and DTAB by increasing of concentration. For C12E1 the surface phase transition is from liquid expanded to liquid condensed state.