An effective cross-sectional area per surfactant molecule at hydrophobic interfaces of aggregates, a(s), in hexagonal (H-1) and lamellar (L-alpha) liquid crystals was calculated in homogeneous and mixed polyoxyethylene dodecyl ether systems as a function of polyoxyethylene (EO) chain length by means of small-angle X-ray scattering. The a, increases with increasing the EO chain length. The a(s) in the mixed surfactant system is considerably smaller than that in the single surfactant system, even if the average EO chain length is the same. The reduction of a(s) is larger than that predicted by ideal mixing of the surfactants. Moreover, if the EO chain lengths of the surfactants are more separated, the a, is smaller. The shapes of surfactant self-organizing structures may be governed by the balance of the attractive and the repulsive forces acting at the hydrophobic interfaces of the aggregates. According to this consideration, the mixing effect of surfactants with the different EO chain lengths on the a(s) in the L-alpha phase was discussed. It is considered that the surfactant molecules are tightly packed in the aggregates since the reduction in repulsion force takes place in the excess EO chain part of the hydrophilic surfactant longer than the short EO chain of the lipophilic one. The lower surface tensions and the better stability of macroemulsions and the large solubilizing capacity of microemulsions result from the mixing effect.