This study evaluates and compares the parameters of equilibrium adsorption from aqueous solutions of river fulvic acids (FA) and peat fulvic acids. Compared to peat FA, the river FA had lower biodegradable dissolved organic carbon (BDOC). We observed a tendency for correlation between change of Gibbs free energy of adsorption and BDOC value. A chlorination dose of 1.4 mg L−1 did not change FA biodegradability and adsorbability. ClO2 treatment produced similar results; ClO2 had a stronger oxidizing influence on FA with low BDOC than on FA with high BDOC. In general, the oxidative effect of O3 was greater than Cl2 or ClO2 and the oxidative effect of ClO2 was greater than Cl2. Qi–Schideman and modified Freundlich equations well described all isotherms of FA on activated carbon F300. The content of low-adsorbed substances in peat FA decreased in the sequence: initial solution – Cl2 – ClO2 – O3. A similar difference was also seen with BDOC content in FA solutions. For peat FA with high BDOC content, the low-adsorbed fraction transformed to a strong-adsorbed fraction. Chlorination and chlorine dioxide treatment decreased low-adsorbed fraction absorbability, although ozone treatment increased it, which corresponded with a change in BDOC and a change in Gibbs free energy of adsorption. Chlorination and ClO2 treatment increased the non-adsorbable fraction, and ozonation substantially decreased the non-adsorbable part of the FA. Treatment by chlorine and ozone increased the concentration range of the strong-adsorbed fraction; chlorine dioxide did not significantly influence this fraction.