According to the temperature-composition phase diagram of Cetyltrimethylammonium chloride (CTAC) andp-phenylphenol (p-PP), which was obtained by DTA measurement of powder mixtures at various composition ratios, it was found that only one complex with a 1/1 molar ratio was available for the CTAC/p-PP system. The single crystal structure of this complex, together with the crystal structure of CTAC, was determined by the X-ray diffraction method. Both crystals have a smectic layer structure stacked along thec-axis. The CTAC (andp-PP) molecules are arranged almost perpendicular to the layer surface. Within the smectic layer of the host (CTAC) crystal, long alkyl chains from both sides of layer surfaces are interdigitated mutually, which makes a hydrophobic region in the middle of the layer. In complex (CTAC/p-PP) crystals, guest (p-PP) molecules were accommodated in the space formed by loosening their interdigitation. The arrangement ofp-PP molecules found in the complex crystal is very similar to that in thep-PP crystal. The thermal behavior of complex crystals is also very similar to that ofp-PP. Therefore, the complex structure seemed to be highly influenced by the guest-guest interaction. We call this type of complex structure a “guest-dependent” structure.