A method and deposition system for increasing deposition rates of metal layers from metal-carbonyl precursors using CO gas and a dilution gas. The method includes providing a substrate in a process chamber of a processing system, forming a process gas containing a metal-carbonyl precursor vapor and a CO gas, diluting the process gas in the process chamber, and exposing the substrate to the diluted process gas to deposit a metal layer on the substrate by a thermal chemical vapor deposition process. The deposition system contains a substrate holder configured for supporting and heating a substrate in a process chamber having a vapor distribution system, a precursor delivery system configured for forming a process gas containing a metal-carbonyl precursor vapor and a CO gas and for introducing the process gas to the vapor distribution system, a dilution gas source configured for adding a dilution gas to the process gas in the process chamber, and a controller configured for controlling the deposition system during exposure of the substrate to the diluted process gas to deposit a metal layer on the substrate by a thermal chemical vapor deposition process.