Photocatalytic (PC) and photoelectrocatalytic (PEC) degradation of small biological compounds such as nucleotide bases were carried out because of the nucleotide bases are the building blocks of large biomolecules, nucleic acids. These small biological compounds, four different nucleotide bases, can be photocatalytically and photoelectrocatalytically degradable, and the degradation efficiencies of PEC method were found to be higher than those of PC method for all compounds investigated. Also, we tried to propose the photocatalytical and photoelectrocatalytic degradation mechanisms of nucleotide bases, but the performance was unsuccessful. However, organic nitrogen in the original compounds was found to be oxidized to either NH3/NH4+ or NO3- or both, depending on the chemical structures of the nucleotide bases and the degradation methods used. Based on both the experimental results and the theoretically calculated frontier electron densities (FED) values of 2FED(HOMO)(2) and FEDHOMO2 + FEDLUMO2, the conclusion can be demonstrated as the reaction mechanisms/pathways of PEC processes differed remarkably from that of PC processes. (C) 2014 Elsevier B.V. All rights reserved.