The electron spin resonance (ESR) spectra of a spin probe, deuterated 2, 2, 6, 6-tetramethyl-4-oxo piperidine-d₁₆-1-oxyl (TEMPONE-D), in the binary system of 3- (isooctadecyloxy) -1, 2-propanediol (GE) and water were measured for structural clarification of a liquid crystal (LC) in the GE/water system. Deuteration of the TEMPONE molecule should contribute to narrow ESR signals. The narrow signals facilitated the analysis of ESR signal patterns and obtaining dynamic information on molecular aggregation. In the system whose weight fraction of water was more than 0.25 (LC+water), two sets of ESR signals of TEMPONE-Dcorresponding to LC of GE and separated water, could be observed sepalately. The exchange rate of TEMPONE-D between the LC of GE and separated water is thus quite slow. Consequently, as noted previously, LC of GE may serve to maintain the reversed hexagonal structureand its strong hydrophobicity, regardless of water content. The present method was found quite useful for examining the characteristics of a molecular aggregation such as LC.