Reversible thermal annealing of YBa2Cu3Ox single crystals in air has been studied by microwave loss measurements. Among the advantages of this contactless technique are the following: Crystals are kept in sealed capillaries throughout cycles of annealing and low-temperature measurement; features are resolved in loss versus temperature measurements that are not revealed by other techniques. Microwave loss is interpreted in terms of the Gittleman-Rosenblum model of fluxon motion in a viscous medium. Narrow temperature ranges are found for the formation of the 60- and 90-K phases. Optimal microwave response is obtained after but a few hours of annealing, indicating the importance of c-axis diffusion.