Enthalpies of solution of five amino acids, glycine, I-alanine, l-valine, l-serine, and l-threonine in aqueous solutions of three quarternary ammonium surfactants, [CnH2n+1(CH3)(2)NCH2CH2OH]Br (n = 12, 14, 16), have been measured at 298.15 K with a microcalorimeter. Enthalpies of transfer of amino acids from water to aqueous surfactant solutions have been derived. It has been observed that, at relatively low concentrations of the surfactant solutions, amino acids are still in the water phase and mainly interact with the hydrophilic head groups of the surfactant molecules. With an increase of the concentration of the surfactant solutions, the microenvironment of amino acids changes, and the molecules might insert into the micelles and interact with the hydrophobic tail groups of the surfactant molecules. The results are discussed in terms of a delicate balance of hydrophobic and hydrophilic interactions and differences in the molecular structure of amino acids.