ZnO crystal growth by chemical vapor transport (CVT) is carried out using CO2 and Zn as a new transport agent. ZnO crystals are grown by the reaction: ZnO(s) + CO(g) = Zn(g) + CO2(g). The increase in CO2/Zn changes the color of as-grown crystals from red to transparent. The color change of the crystals is due to the oxygen partial pressure during the CVT. The red color of crystals is attributed to defects caused by the shift of stoichiometry to zinc-rich (oxygen vacancies, zinc interstitials, or their complexes are created). All of the grown crystals are polycrystals by using the ordinary CVT ampoules. Single crystals are grown reproducibly by using a seed and a CVT ampoule with modified configuration. (C) 2007 Elsevier B.V. All rights reserved.