Properties and performance of materials are closely connected. In order to obtain piezoelectric and lasing optical quality, ZnO has to be free of defects and highly crystalline. Instead, conductivity depends upon such defects, making it not trivial to aim at a specific set of properties in a single step. In this regard, we studied in situ the effect of temperature as an additional knob to finely control such properties. In this contribution, plasma enhanced atomic layer deposited (PE-ALD) zinc oxide (ZnO) layers, deposited between 25 degrees C and 250 degrees C, were studied in situ during annealing in air, and the opto-chemical and structural characteristics of the oxides were followed as a function of temperature. In situ spectroscopic ellipsometry (SE) and X-ray diffraction (XRD) were adopted to identify temperature windows where major structural and optical changes in the material occurred. Two temperature regions were identified for the effusion of adsorbed gases and minor structural rearrangements (180-280 degrees C) and for the growth/coalescence of ZnO crystals and its densification (360-500 degrees C). The results were corroborated by ex situ SE, XRD, UV-Vis and X-ray photoelectron spectroscopy. The in situ study revealed differences among the ZnO layers deposited at different temperatures, giving additional insights on the material properties deposited by PE-ALD.