The effect of low-temperature annealing on the superconducting transition temperature and electrical resistance of single crystals ofYBa2Cu3O7−&dgr;with an oxygen deficiency&dgr;=0.5–0.6quenched from temperatures of 620–650 °C is considered. The transition temperature increases during annealing, while the resistance decreases. The isothermal relaxation of resistance is measured and used for estimating the activation energy of the relaxation process, which coincides with the activation energy of oxygen diffusion. The obtained results are attributed to oxygen ordering in Cu-O planes without changing its concentration. The observed step form of resistive transitions to the superconducting state is interpreted as the formation of clusters characterized by different oxygen concentrations and ordering. The cluster size is estimated. ©1997 American Institute of Physics.