Abstract In the present work, all the by-products formed during the urea electrooxidation (UEO) by Ni(III) in alkaline medium are identified and quantified for the first time ( e.g., OCN- , CO_3^{2-}, NH_4^+, N_2, NO_2^- ). Complete mass balances are simultaneously established in both aqueous and gas phases for large urea conversion rates (>80%). The results provide clear evidence (with a maximal deviation of 2%) that, in addition to N_2 , N-overoxidized by-products are produced during the UEO. Electrolyses conducted with human urine samples under identical operating conditions result in different distributions of N and C-by-products. In particular, the formation of formic and oxalic acids in these latter experiments suggests the presence of different and/or additional mechanistic pathways during the UEO process. The amount of H_2 generated at the cathode is also quantified, showing a reduction of 30% in energy consumption when compared with water electrolysis. Finally, this study assesses the energy efficiency of UEO with synthetic urea and real-matrix effluents, thereby contributing to the development of this sustainable process, which enables energy recovery from waste.