Potentiometric titration is a common way to characterize dissolved organic matter (DOM) reactivity. However, due to the sensitivity of pH electrodes, it is necessary to work with high DOM (> 1 g/L) concentrations that are unrealistic compared to DOM concentrations found in natural waters (0.1 to 100 mg/L). To obtain reactivity data for concentrations closer to environmental values, a new approach is considered. UV-vis spectra and spectrophotometric titrations of DOM [1] are used to estimate changes in reactivity as function of pH with the help of differential absorbance spectra variations of a diluted solution of humic acid used as a DOM analogue. There is a good correlation between spectrophotometric and potentiometric results, at different ionic strength. Spectrophotometric titrations are then used to determine the changes of humic acid properties after adsorption onto a mineral surface (i.e. aluminium oxide). [1] Dryer, Korshin & Fabbricino (2008) Environ. Sci. Technol. 42, 6644-6649.