Carbon is a common unintentional impurity in oxide semiconductors. We use hybrid density functional theory to calculate the electronic and structural properties of carbon impurities in ZnO, In 2 O 3 , and Ga 2 O 3 -materials that are used as transparent conductors. In each of these semiconducting oxides, we find that carbon is most likely to occupy the cation site under most electronic and chemical potential conditions. In ZnO, C Zn acts as a shallow double donor and exhibits large local breathing-mode relaxations. In In 2 O 3 and Ga 2 O 3 , C acts as a shallow donor and moves off the cation site to become threefold oxygen coordinated. In all three oxides, C cation exhibits modest formation energies, indicating that these species will be likely to incorporate. Our results indicate that C impurities are suitable donor dopants in these oxides and will contribute to background n-type conductivity if unintentionally present.