The temperature and enthalpy vs composition diagrams of the binary system [(1 - x) n-C9H21CO2H + (x) n-C9H21CO2Tl, where x = mole fraction] were determined by means of differential scanning calorimetry, differential thermal analysis, and optical microscopy. Besides the usual regions of phase separation, miscibility was observed in isotropic liquid, liquid crystalline, and solid phases. This binary system displays the formation of: (1) a 1:1 molecular compound which melts incongruently at T = 322.2 +/- 13.1 K, Delta(fus)H(m) = 50.7 +/- 0.5 kJ.mol(-1); (2) an eutectic at x = 0.125 and T = 302.0 +/- 0.1 K, Delta(fus)H(m) = 28.3 +/- 0.3 kJ.mol(-1); (3) a smectic A lyotropic mesophase occurring at T = 373.5 +/- 0.1 K over the composition range x > 0.770; (4) a solid solution phase occurring over the composition x > 0.913, and 315.3 K < T < 404.2 K.