Measurement of free Zn2+ activity is important for understanding the mechanisms controlling Zn2+ solubility in soils. Free uncomplexed Zn2+ activities were measured using the chelation method in 12 uncontaminated soils and eight contaminated soils. The measured Zn2+ activities ranged from 10−9.97 to 10−6.49M for the uncontaminated soils, and 10−8.11 to 10−2.26M for the contaminated soils. They were inversely related to soil pH, with a correlation coefficient of 0.96 for the uncontaminated soils and 0.89 for the contaminated soils. The measured Zn2+ activities and pH for the uncontaminated and contaminated soils can be expressed as log(Zn2+) = (5.90 ± 0.24) − 2 pH and log(Zn2+) = (9.38 ± 0.74) − 2 pH, respectively. The measured Zn2+ activities in the uncontaminated soils were near that of franklinite (ZnFe2O4). The measured Zn2+ activities in the contaminated soils were close to those of Zn2SiO4 or ZnCO3 depending on the contaminated soils. Different solid phases may control Zn solubility in different soils depending on the sources of contamination and the soil.