Copper (I) oxide thin films are deposited on quartz substrates by DC magnetron reactive sputtering. This study examines the effect of post-annealing on their optoelectronic properties in detail. The films are grown by sputtering from copper in an atmosphere of argon and oxygen. The substrate temperature is held at 200 °C, while annealing in ambient atmosphere has been carried out between 100 and 600 °C. X-ray diffraction analysis, Raman and UV–Vis spectroscopy, and four-probe measurements were used to characterise the films. XRD indicates that deposited Cu2O has a preferred orientation of (110). Post-annealing did not show any measurable conversion to copper (II) oxide until about 500 °C, and the process was incomplete even at 600 °C. The highest conductivity is observed in the sample post-annealed at 100 °C. These results are of substantial technological importance for using Cu2O for a variety of applications, including transparent solar cell fabrication.