BACKGROUND: ZnO nanorods, which have a wide range of applications, were grown on a Si substrate by low temperature hydrothermal synthesis. An understanding of the reaction mechanism of ZnO nanorods is crucial to control their growth kinetics. Therefore, the effect of OH-concentration in a zinc sulfate solution on the growth of ZnO nanorods was investigated in order to better understand the growth mechanism of ZnO nanorods. RESULTS: The growth rate and diameter of ZnO nanorods were increased by increasing the pH of the zinc sulfate solution from 10.4 to 10.6, and the highest growth rate, 850 nm h(-1), was observed when the nanorods were grown in a solution with a pH of 10.6. However, further increase in the pH of the solution decreased the growth rate, due to the simultaneous dissolution of ZnO nanorods by OH-. The amount of OH-consumed by the dissolution of ZnO and other subreactions was minimized in a solution with a pH of 10.6. The intensity of c-axis (0001) orientation was the highest in the solution with a pH of 10.6. CONCLUSION: It is concluded that the concentration of OH- plays a critical role in the hydrothermal synthesis of ZnO nanorods, and that the suppression of OH- subreactions increases the growth rate of ZnO nanorods. From the changes in the length and diameter of ZnO nanorods with synthesis time, it is suggested that the nucleation of ZnO nanocrystals occurs in the first 30 min, from which the growth of nanorods then proceeds. (C) 2007 Society of Chemical Industry.