A theoretical study on the mechanism of the reaction between o-phenylenediamine and phenylpyruvic acid was performed using computer assisted molecular modeling, in order to explain the specially low yields observed in the attainment of the corresponding 3-benzyl-quinoxalin-2(lH)-one. Theoretical calculations and experimental data (UV-vis, ¹H-NMR and fluorescence spectra) were analysed, arriving to interesting conclusions.