The effects of Si additions on the mechanical properties of molybdenum carbide and zirconium carbide thin films have been examined in this study. Mo-Si-C and Zr-Si-C thin films were deposited by magnetron co-sputter deposition using carbide targets. The films were deposited onto Si substrates at 350C for Mo-Si-C and 450C for Zr-Si-C. The structure, texture and crystallinity of the films were analyzed using x-ray diffraction, and high-resolution transmission electron microscopy was used to study film microstructure. X-ray photoelectron spectroscopy (XPS) was used to determine film composition and also to assess the chemical state of the carbon, as well as oxygen impurities, in the films. For Mo-C films with no or little Si, the structure was hexagonal; as the Si content increased, a transition to a face-centered cubic structure was observed. At the highest Si contents (10-14%) the structure was essentially amorphous. The TEM studies showed as Si was added, the grain size was reduced and confirmed the amorphous structure at the high Si contents. The texture of the films also evolved with Si content, showing oriented films at low Si but random grain structures (continuous Debye rings) at intermediate Si concentrations. The mechanical properties of the films were analyzed using nano-indentation, and showed little change in hardness over the entire range of film compositions. Zr-Si-C films were similarly studied, and showed a stronger tendency for amorphization as Si was added. The nano-indentation tests on these films showed a peak in hardness near 18% Si, where the hardness of near 30 GPa was more that 50% higher than for ZrC films alone, while the film modulus increased only slightly. Initial TEM studies show that films with added Si have a higher density, which may be a factor contributing to the observed hardness increase.