The thermodynamics of long aliphatic chain alkylamine aggregation in aqueous solution was studied by isothermal titration calorimetry (ITC). Protonated alkylammonium cations with linear aliphatic chains of 10-14 carbon atoms were fully soluble in aqueous solution at the beginning of titration, but practically insoluble after deprotonation by titrating with sodium hydroxide. The alkylamines aggregated and precipitated during the reaction, enabling direct measurement of the enthalpy of aggregation. The enthalpy of aggregation became increasingly exothermic upon increasing the chain length. Hydrophobic aggregation was enthalpy-driven and entropy-opposed for alkylamines with 12-14 carbon atoms at room temperature. Direct observation of hydrophobic aggregation by ITC at constant temperature and pressure provided more accurate thermodynamic parameters than obtainable from van't Hoff analysis. Aggregation into liquid or solid phases could be distinguished by ITC, but not by van't Hoff analysis of alkylamine solubility data.