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[Display omitted] • DCF degradation was investigated via UV, UV/H 2 O 2 , and UV/FC at 265, 285 and 310 nm. • The degradation depends upon the absorption of the contaminant and the oxidant. • The main reaction mechanisms for UV/H 2 O 2 and UV/FC were proposed. • The electricity demand is similar for all treatments due to the oxidants' cost. • No significant synergy was found for the dual-wavelength system (265 + 285 nm). In this study, the degradation of diclofenac (DCF), a frequently detected non-steroidal pharmaceutical, was evaluated by using UV-B and UV-C (265, 285, and 310 nm) light-emitting diodes (LEDs) alone and in combination with hydrogen peroxide (UV/H 2 O 2) and free chlorine (UV/FC). The degradation of DCF followed a pseudo first-order kinetic, and their trend reflected the pattern of the molar absorption coefficients of the DCF and the oxidants. A positive synergistic factor was found for the UV-LED driven advanced oxidation processes in almost all cases, but despite the higher degradation rates, the overall electricity demand is similar to UV alone due to the oxidants' energy cost. The rigorous kinetic degradation mechanisms at different wavelengths were proposed for the two processes, UV/H 2 O 2 and UV/FC, where the predicted values were respectively k HO = 9.12 · 109 M−1 s−1 and k Cl = 1.30 · 1010 M−1 s−1. No significant synergy (p > 0.05) was found for the dual-wavelength system (265 + 285 nm), and the time-based constants in all cases changed linearly with lamp intensity. Finally, dissolved organic carbon and phytotoxicity analysis revealed low mineralisation (around 20–30%) associated with the formation of stable dimers and a decrease in toxicity towards tomato and radish seeds. In the main, this work shows the great potential of implementing wavelength-specific LEDs in water treatments and effectively designing the reactor playing with adjustable intensities and kinetic degradation rates. [ABSTRACT FROM AUTHOR]