We present a theoretical explanation for experimental results obtained recently regarding dimeric surfactants. The nonmonotonic dependence of the specific area at the air/water interface on the spacer carbon number is accounted for. In addition, understanding the role of spacer carbon number at the air/water interface can elucidate the shapes of aggregates formed in the aqueous solution. The attractive and repulsive interactions of the surfactant molecules and the conformational entropy of the spacer chain are found to be dominant factors in determining this dependence. On the other hand, hydrophobic repulsion of the spacer from the water surface does not seem to play an important role, if any, contrary to what has been previously suggested.