Using a Mark IV surface force apparatus, adsorption of dodecylamine hydrochloride has been studied at pH 5.7 in the presence and absence of octanol. With amine alone, the repulsive forces between mica surfaces are reduced due to charge neutralization and attractive hydrophobic force; however, the latter is of relatively short range with decay lengths in the range of 1.2-1.4 nm. In the presence of amine and octanol, long-range hydrophobic forces are observed, the largest decay length being 6.8 nm. The appearance of the long-range hydrophobic force in the presence of octanol may be attributed to the coadsorption of the neutral surfactant in the monolayer of dodecylammonium ions on mica. The increased hydrophobic force due to the coadsorption of octanol is also manifested by the improved floatability of silica in dodecylamine solutions.