Positron lifetime measurements were performed in TTAB(Tetradecyl trimethyl ammonium bromide)/water/pentanol ternary systems prepared by adding varying amounts of pentanol to different mother solutions of TTAB/water system having fixed TTAB concentrations. Besides delineating various phase boundaries as obtained by other conventional techniques, positron annihilation parameters were also found to be sensitive in detecting two kinds of lamellar structures in the otherwise considered to be a single liquid crystalline D phase of the system. The existence of such lamellar structures has been demonstrated by a change in the trend of o-Ps lifetime parameter when the system passes from one type of lamellar structure to the other type. The results of such a finding are presented in this paper.