The thermal properties of a smectic liquid crystal in the (L(alpha) + H2O) phase of dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylglycerol (DMPG) were investigated as a function of temperature in the region from 24 to 40 degrees C by the ESR spin probe method using the doxylstearic acids, 5NS, 7NS, and 12NS. The molecular flexibility and homogeneity of these bilayer assemblies were analyzed from the ESR spectra observed at different labeled positions relating to the hydrocarbon chains in bilayers. In the case of a DMPC dispersion, the hyperfine splitting constants for 5NS abruptly increased and then became almost constant above about 29 degrees C, while the symmetry parameters of 12NS and the b/a parameters for 5NS decreased steeply with increasing temperature up to about 29 degrees C and then their values were almost constant. On the other hand, in the case of the DMPG dispersion, the hyperfine splitting constants of 5NS decreased with increasing temperature up to about 31 degrees C, and then they became constant. As a result, it has been confirmed that the temperatures at which the temperature dependence of these ESR parameters changed drastically coincided with the critical temperatures, T* (29.0 degrees C for DMPC and 31.7 degrees C for DMPG), at which the thermal character of these bilayers in (L(alpha) + H2O) phases has been found to change from our earlier surface chemical and calorimetric data. Thus, by the present ESR study, the presence of an intermediary phase (L(alpha)’) between the P-beta’ phase and the L(alpha) phase has been revealed for the temperature region from the gel-liquid crystal transition temperatures, T-M (23.7 degrees C for DMPC and 23.3 degrees C for DMPG), to the critical temperatures (T*).