A general adsorption model based on partial dissociation was developed for carboxylic acids. The model was applied to the adsorption of nonanoic acid at the air/water interface. Two cases were selected for experimental verification: acid-only and acid with a constant Na + OH – concentration. The model was applied simultaneously at both conditions, and the hydrophilicity of the ionic states was quantified by the adsorption constants, K A and K AH . It was found that the adsorption constant for the acidic group is significantly higher than that for the carboxylate group, K AH /K A ∼ 272. The model lays important groundwork for modeling and predicting carboxylic/carboxylate adsorption.