Potentiometric and counter-ion adsorption techniques were used to study the effects of pH, electrolyte concentration, and phosphate treatment on charges associated with a Typic Gibbsihumox and a Typic Hydrandept. The methods did not agree on either the isoelectric points or the net charges associated with the soils. Furthermore, neither method provided data at pH 7 that are close to those obtained during routine cation exchange capacity determinations. The Gibbsihumox fit the definition of a constant surface potential colloid in terms of the combined Nernstian-Stern theory treatment better than the Hydrandept did. The latter fit the definition well when its surfaces acquired larger magnitudes of negative charge due to KH2PO4treatment.