Ga and N co-incorporated ZnO thin films [ZnO:(Ga:N)] with reduced bandgaps were deposited by co-sputtering at different N(2) gas flow rate in mixed N(2) and O(2) ambient at room temperature followed by postannealing at 500 degrees C in air for 2 h. We found that all of the ZnO:(Ga:N) films exhibited enhanced crystallinity which can suppress the recombination rate between the photogenerated electrons and holes. However, phase segregation of Zn(3)N(2) occurred in ZnO:(Ga:N) thin films in nitrogen-rich sputtering ambient. We found that ZnO:(Ga:N) thin films without phase separation of Zn(3)N(2) exhibited much better photoelectrochemical (PEC) response, due to the reduced bandgap and better crystallinity. Our results suggest that growth conditions must be controlled carefully to avoid phase separation in Ga and N co-incorporated ZnO thin films to improve PEC response. (C) 2011 Elsevier B.V. All rights reserved.