Cation exchange capacity (CEC) of the 0–5 cm depth of forest soils increased with the square root of solution ionic strength over the experimental range of 6 to 96 mM. Percentage increases in CEC were positively correlated with percentage organic carbon; for mixed hardwood forest soils, increases were38%for soils with 3.3% carbon, and 105% for soils with 7.4% carbon when ionic strength was varied over the full experimental range. When analyzing soils with constant‐potential surfaces, both pH and ionic strength must be controlled, preferably at or near levels found in the field, in order to provide interpretable measures of CEC.