An investigation of radio-frequency plasma-deposited amorphous hydrogenated boron films was conducted in order to determine the influence of the substrate temperature T-S and d.c. self-bias U-SB on the physical properties and film stability. We found three different regions of stability depending on T-S and U-SB. Films prepared at high d.c. self-bias and substrate temperature are mechanically unstable due to internal stress. They peel off during the deposition process or on first contact with the ambient atmosphere. Films deposited at low self-bias and substrate temperature were found to be chemically unstable. Exposed to the ambient atmosphere, they undergo chemical changes and incorporate large amounts of oxygen and carbon. These two different regions of instability are separated by chemically and mechanically stable films. The chemical composition and the structural properties of chemically stable and unstable films were determined by Fourier transform infrared spectroscopy, X-ray induced photoelectron spectroscopy and ion beam analysis. These measurements show that chemical stability correlates with the boron density. Chemically stable films reveal densities of more than 68% up to 99% of the density of crystalline boron. In general, elevated substrate temperature and ion energy cause densification of the films and increasing internal stress. Densification leads to chemical stability, while internal stress is the reason for mechanical instability.