The behaviour of α,ω-diacid and α-monoacid guest molecules (with carbon atoms ranging between 6 to 22) in the lamellar and hexagonal mesophases of dipotassium thapsate (KOOC(CH2)14MCOOK)-water mixtures have been investigated, using deuterium NMR spectroscopy. Addition of short diacids to either the lamellar or hexagonal phases results in phase separation and essentially no incorporation into the mesophase structure. The deuterium spectra of the guest molecules exhibit characteristic patterns which are qualitatively interpreted in terms of their ordering properties. In no case were separate signals observed due to folded diacid guest molecules which have their carboxyl groups on the same side of the interface.