Hall-effect, photoluminescence (PL), and electron paramagnetic resonance (EPR) measurements have been performed in single-crystal ZnO samples annealed in air at 25, 550, 750, and 950 degreesC, for 30 min each. A 37 meV donor is dominant in the unannealed sample, but nearly disappears during the higher temperature anneals, and is replaced by a 67 meV donor. The 37 meV donor is responsible for a donor-bound-exciton PL line at (3.3631 +/- 0.0002) eV, which is dominant in the unannealed sample. The EPR measurements show that N-O centers appear in the 750 degreesC and 950 degreesC anneals, and they are probably responsible for an increase in the acceptor concentration found from the Hall-effect results. A PL emission at (3.3570 +/- 0.0002) eV, assigned in the literature as an acceptor-bound exciton, may involve N-O complexes.