The most frequently used theories of surfactant adsorption phenomena treat adsorbing molecules as structureless entities. However, surface active molecules consist of hydrophillic headgroups and hydrophobic chains. Therefore, surfactants have additional degrees of freedom associated with conformations of the chains. An unrestricted hydrophobic chain can assume practically any conformation at the interface. Adsorption of other surfactant molecules provides restriction to the number of possible conformations changing the conformational free energy. This change has to be reflected in the adsorption isotherm. We present a simple model of adsorption of soluble surfactants at the air/solution interface, taking explicitly into account the conformational statistics of hydrophobic surfactant chains. The model is applied to describe adsorption isotherms of homologous series of dimethyl-n-alkyl phosphine oxides with the alkyl chain length of 7-13 carbon atoms.