A novel soft-solution technique is used to grow epitaxial Zn0.98CO0.02O films on the c-plane (0001) of a single-crystal sapphire substrate. Annealing the films in precisely controlled atmospheres shows that the ferromagnetism is not linked to the carrier-concentration density. Undertaking cyclic annealing studies in the presence or absence of Zn vapor (see figure) shows that Zn interstitials but not oxygen vacancies are the critical defects for inducing ferromagnetism.