In this paper the physicochemical and spectroscopic properties of humic acid (HA) obtained from composted walnut shell of Juglans regia are described. The influence of oxygen on UV-induced photodegradation of HA was investigated using absorption, fluorescence and electron paramagnetic resonance (EPR) spectroscopy. Fluorescence spectroscopy revealed two chromophoric centers. The center featuring long wavelength emission at 510 nm and excitation peaks at 440 and 460 nm appears more resistant to UV and oxygen than the center featuring an emission maximum at 450 nm and excitation peaks at 260 and 350 nm. The latter center can be related to condensed aromatic rings, unsaturated bond systems and electron-withdrawing groups such as carbonyl and carboxyl groups susceptible to oxidation processes. The gradual increase in values of the colour coefficients Q4/6, Q2,6/4 and Q4/5.2 and the concomitant decrease in absorbance for all wavelengths suggested the occurrence of a gradual photodegradation process involving the formation of phenolic and simpler carboxylic acids during irradiation. These modifications would lead to a decrease in molecular weight and lower condensation of HA macromolecules. The high semiquinone radical content measured by EPR spectroscopy is indicative of high concentration of quinoid structures in HA from walnut shells. The sensitivity of non-irradiated samples to the oxygen level also suggested the presence of alcoholic-hydroxyl, phenolic-hydroxyl and carboxylic functional groups susceptible to oxidation in HA.