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The dielectric dispersion in the frequency range 2–800 MHz of the zwitterionic dipolar phosphocholine head group has been measured from −20 to +60°C for DMPC depending on the amount of bound water. It is found that the head group rotation shows a sudden change at the gel‐fluid phase transition, 1. the rotation frequency becomes much higher and 2. the related dielectric response becomes much more pronounced in the fluid phase. Below 15 H2O molecules bound per lipid molecule the head group rotation frequency decreases with decreasing number of bound water. This observation reveals for the first time that there is a correlation between hydration dependence of the chain melting temperature and head group rotation. The thermal activation of the head group rotation in the fluid phase made us conclude that three hydrogen bonds have to be broken in order to rotate the head group.

The dielectric response (1 MHz + 10 GHz) and the structure of a water-oil emulsion were studied as a function of temperature. It was observed that the water component causes the dielectric loss of the emulsion in the crystalline gel phase at low temperatures. However, the dielectric loss decreases discontinuously at the gel-microemulsion transition near 60 "C. At higher temperatures, i.e. in the microemulsion phase of the systems, the dipole relaxation becomes very slow, similar to that of ice. This behaviour is explained by an enhanced connectivity of the hydrogen bonds of water in the micelles of the microemulsion. We conclude that the water in the micelles behaves like frozen water, even at temperatures above 60 "C.