Partial molar heats of solution of both water and triethylamine have been measured at 15° over the whole range of composition in the water-triethylamine system. Integral heats of mixing have been calculated from these results and compared with earlier results obtained by different methods. Vapor pressure data have been used in calculating partial molar free energies and have been combined with partial molar heats to yield partial molar entropies, which indicate that triethylamine is a “structure maker” in dilute aqueous solution. Heats of solution of several nonelectrolytes have also been measured in water-triethylamine systems. Results are discussed in relation to heats of solution in water-ethanol and the effects of third components on critical mixing in water-triethylamine systems.