This paper presents a comparative study on the effect of transition metal and rare-earth doping on GaN and ZnO by metal organic vapor phase epitaxy (MOVPE). Bulk ZnO single crystals were also analyzed in this study. Structural and optical characterization of these materials did not show the presence of any secondary phases or significant degradation of optical property. Magnetization measurements revealed different magnetic behavior for ZnO-bulk crystals and thin films that have been doped with the same transition metal, suggesting that the magnetic properties are dependent on growth conditions. Further, it was found that with co-doping and annealing the magnetization strength and charge state of Ga1-xMnxN can be altered. A comparative analysis on the effect of silane doping on Ga1-xMnxN and Ga1-xFexN revealed that nanoclusters caused by spinodal decomposition are the likely cause of the observed ferromagnetism. This paper presents the first report on MOVIRE-grown Ga1-xGdxN, Room temperature ferromagnetism is shown for Ga1-xGdxN and its magnetization strength increases with co-doping. The results obtained in this study are promising, as it has been shown that the magnetization can be controlled and the transition-metal charge state can be changed by modifying the MOVPE growth conditions: applying slow growth rates, antisurfactants and employing co-dopants. (C) 2008 Elsevier B.V. All rights reserved.