The temperature effect on the magnitude of adsorption was used to explain the mechanism of adsorption of gadolinium on silica at very low concentrations. Standard enthalpy of adsorption of gadolinium equals 36 kJ mol(-1) for a total Gd concentration of 2 x 10(-8) mol dm(-3) and 67 kJ mol(-1) for 2 x 10(-5) mol dm(-3). This result confirms the hypothesis that the Gd adsorption at low initial concentration is governed by formation of strong ternary surface complexes involving anionic impurities.