Knowledge of physical, chemical, and biological processes affecting organic contaminants in soils is essential to develop remediation technologies and assess risk from alternative technologies. Of these processes, sorption is one of the most important in soils because it regulates other processes and hence fate, transport and removal of contaminants. The objectives of this research are: 1) to evaluate the thermodynamic parameters and bonding strength for pentachlorophenol (PCP) onto montmorillonite at ionic strengths from 0 to 0.3 molal; and, 2) to test the hypotheses that adsorption is through positive ion pair formation (e.g. CaPCP +), and chemical bonding of phenolate. Calorimetry, Gas-liquid chromatography, Fourier transform infrared spectroscopy, and X-ray diffraction were used in the experiment. The adsorption of PCP onto montmorillonite was strongly influenced by the type of cations and the ionic strength. PCP molecules were mainly adsorbed on the edge of montmorillonite. The standard enthalpy changes (ΔH°) of PCP adsorption on Ca-montmorillonite ranged from −24.3 to −38.0 kJ/mol and were most negative at 0.03 molal solution ion concentration, as was ΔG° which ranged from −15.4 to −17.6 kJ/mol, and ΔS° which fell between −29.9 and −68.4 J/K mol. The highest value of lnK O was 7.11, and was obtained at 0.03 molal. Our experimental results are consistent with the above hypotheses concerning adsorption, and multiple mechanisms of adsorption (Coulombic interaction, physi and chemisorption) have been inferred.