Photocatalytic degradation of nitrobenzene (NB) and an azo dye (namely azophloxine) was carried out using zinc oxide (ZnO) nanoparticles which were prepared by electrochemical method using zinc electrodes in aqueous oxalic acid solution. Taguchi methodology was applied for studying the effects of four experimental parameters namely catalyst dosage (g/1). time of reaction (h), pH and H2O2 dose. Degradation efficiency of NB was found to increase with an increase in H2O2 concentration and decrease in catalyst dosage and pH. Maximum NB degradation efficiency of 98% was observed at catalyst dosage=l g/1, pH=3, H2O2 to C6H5NO2 ratio of 7 mol/mol after 5 h of treatment. Similarly, maximum dye degradation efficiency of 58% and color removal efficiency of 78% was observed at catalyst dosage=l.25 g/1, pH=4, H2O2 concentration of 8 mmol/1 in 5 h. Overall, prepared ZnO nanoparticles performed better for NB degradation in comparison to azo dye degradation. © 2016 Scientific Publishers. All rights reserved.