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The adsorption behavior of 12-phosphotungstic acid (12-HPW) on silica gel in aqueous solution was studied by a batch method. It was assumed that the adsorption of 12-HPW on silica gel is due to two species: One is (Si OH 2 ) + •(H 2 PW 12 O 40 ) − and/or (SiOH 2 ) + •(H 5 P 2 W 18 O 62 ) − , and the other is the bulk 12-HPW species. The two species adsorb by chemisorption and physisorption, respectively. A model is proposed to correlate the saturated chemisorption capacity limit with the molar percentage of (SiOH 2 ) + •(H 2 PW 12 O 40 ) − , that is, a q Lc −z 1 equation. The adsorption equilibrium data (q e −C e isotherms) were measured and fitted by several models, namely, the Langmuir, Freundlich, and Dubini−Radushkevich (D−R) models, and the mean feature concentration (C e ′) was determined at different temperatures to distinguish chemisorption from physisorption. The values of the correlation coefficient (R 2 ) show that the Freundlich model is more suitable than the Langmuir model. From the D−R model, the experimental saturated chemisorption capacity (q Se ) was obtained, which suggests that (SiOH 2 ) + •(H 5 P 2 W 18 O 62 ) − hardly exists during the adsorption process (z 1 → 1). The values of the mean free energy (E) calculated from the D−R isotherm equation indicate that the adsorption of 12-HPW on silica gel occurs by sequential chemical and physical mechanisms. The thermodynamic parameters ΔG 0 , ΔH 0 , and ΔS 0 for the chemisorption and physisorption processes indicate that the overall adsorption is spontaneous and that higher temperatures contribute to chemisorption, whereas physisorption is favored at lower temperatures.