The corrosion properties of thin films fabricated from the alloys Mg50Zn5Ca and Mg35Zn5Ca by magnetron sputter deposition were investigated in phosphate buffered saline solution (PBS). Microstructural investigations revealed that the deposition conditions have a huge influence on the thin films' microstructure. Such the fabrication of polycrystalline microstructures with precipitates was possible as well as amorphous films. Corrosion tests indicated a significant decrease of the corrosion rate from nearly 80 to 3.5 μm/month for Mg50Zn5Ca and from about 40 to 4 μm/month for Mg35Zn5Ca, respectively upon the change of the microstructure from polycrystalline with precipitates to amorphous.