Ten soil samples were used to measure free Zn²⁺ in soil pore water obtained from soil treated previously with sewage sludge. The samples were chosen to cover soils having different enrichment rates of heavy metals to sewage sludge. The cation exchange resin method was used to measure free Zn²⁺ in soil pore water. Soil total Zn values ranged from 87 to 282 within an average 165 mgkg^-1.  DTPA, NH₄NO₃ extractable-Zn, and pore water soluble Zn represented approximately as average 12, 0.91, and 0.73 % from the total, respectively. Pore water Free Zn²⁺ represented approximately 0.17, 1.46, 26, and 37% from total, DTPA, NH₄NO₃ extractable-Zn, and soluble Zn, respectively. The Zn activities were very low, ranged from 10^-6.5 to 10^-8.01 M, and were inversely related to pH, with a correlation coefficient (R² = 0.95; P < 0.01). The activity of Zn²⁺ in the soil could be predicted from the equation:   Log (Zn²⁺)  = 6.49 – 2pH The measured Zn²⁺ activities were compared to the solubility of various Zn minerals using a -log Zn²⁺ vs. pH solubility diagram. The diagram suggests that zinc ferrite (ZnFe₂O₄) in equilibrium with soil-Fe is a likely solid phase controlling (Zn²⁺) activities in these soils.