The effects of the addition of crown ether 1,4,7,10,13,16-hexaoxacyclooctadecane on the structure of aqueous solutions of surfactants sodium dodecyl sulfate and dodecyltrimethylammonium bromide have been studied by small angle neutron scattering. By modeling the scattering intensities, it was possible to derive, simultaneously, both structural properties and information on the distribution of crown ether between the micellar and aqueous phases. In the case of sodium dodecyl sulfate, an appreciable amount of crown ether was found to be localized in the micellar phase, though it was not possible to establish whether it was in the core or in the shell; there was no evidence of crown ether localization in dodecyltrimethylammonium bromide micelles. The above observations indicate that although, at least in principle, the crown ether methylene groups could interact hydrophobically in the micellar core with the surfactant alkyl chains, the ability of crown ethers to form metal complexes might also lead, in sodium dodecyl sulfate solutions, to electrostatic interactions in the external palisade between the crown ether-sodium ion complex and the surfactant head groups. The absence of crown ether from dodecyltrimethylammonium bromide micelles indicates that the macrocyclic molecules interact with sodium dodecyl sulfate micelles via the formation of a complex between the sodium ion and the crown ether and rules out the possibility of hydrophobic interaction in the micellar core.