Surface modification is an effective way to enhance adsorption of pollutants by soil. In this study, we investigated the individual adsorption of cadmium ion (Cd 2+) and phenol and also in combination by the clay layer of a loessial soil treated with the amphoteric modifier, duodalkylbetaine (BS-12). Three levels of BS-12 modification were compared in this experiment: (1) unmodified soil (CK), (2) modification with an amount of BS-12 equivalent to 50% of the soil's CEC (50BS) and (3) modification with an amount of BS-12 equivalent to 100% of the soil's CEC (100BS). Cd 2+ adsorption was 0.92–1.70 times higher in the amphoteric modified soil compared to unmodified soil. Adsorption isotherms for Cd 2+ displayed a L1-type shape. Phenol adsorption was 1.25–4.35 times higher in the amphoteric modified soil compared to the unmodified control. The adsorption isotherms of phenol on amphoteric modified soils were generally linear, but changed to L1-type isotherms for modified soil in the Cd 2+ + phenol treatment at 40 °C. The results clearly showed that amphoteric modified soil had the ability to simultaneously adsorb Cd 2+ and phenol. Cd 2+ adsorption by the amphoteric modified soil was related to the initial concentration of Cd 2+ in the supernatant. Cd 2+ adsorption in the 100BS treatment exceeded adsorption in the 50BS treatment when Cd 2+ initial concentrations were higher than approximate 200 μg mL −1. Phenol adsorption by modified soils decreased in the order: 100BS > 50BS > CK and was primarily determined by the surface hydrophobicity of the soil. For the unmodified soil, total adsorption in the Cd 2+ + phenol treatment was slightly lower compared to treatments that contained only Cd 2+ or phenol. This indicated an antagonistic effect between the adsorption of Cd 2+ and phenol, which was reduced after amphoteric modification. A comparison of temperature effects on Cd 2+ and phenol adsorption indicated that Cd 2+ was both physically and chemically adsorbed by the amphoteric modified soil, but phenol was primarily adsorbed physically.