The heavy refractory metals and alloys Molybdenum (Mo), Molybdenum – Silicon – Boron (Mo–Si-B; “MoSiBor”), Tungsten (W), Tungsten – Copper (W-Cu), Tungsten – Nickel – Iron (W-Ni-Fe; “Densimet D 176 and 185”) and Tungsten – Nickel – Molybdenum - Iron (W-Ni- Mo-Fe; “Densimet D2M”) were pack-treated at 1100°C with Silicon - powder to form siliconized zones and/or intermetallic phases which are intended to be more oxidation resistant than the plain base materials. These materials (especially the W-based ones) are used at ambient conditions as counterweights, radiation shields etc. because of their high density as well as at high temperatures (600 – 900°C) as metal forming tools, electrodes etc. because of their refractory metal content. In both areas of conditions oxidation of the plain materials occurs and leads to lower functionality or destruction. A suitable oxidation test has been defined to check the presumably enhanced oxidation resistance of the pack-treated materials: an isothermal high temperature oxidation test at 700 and 900°C for one week. At these conditions all untreated materials would have been more or less strongly oxidized. Improved oxidation resistance could be found for the materials with pack-cementation treatment except for sintered Tungsten (92% dense), sharp etched D 185 and D 176 at 900°C and Tungsten – Copper at both temperatures. More stable and dense superficial oxides were formed which led to decreased oxidation rates and could help to increase functional stability and the lifetime of the components. Different pack-treatments e.g. with chromium or silicon plus chromium could improve the behaviour of the materials which failed within this work.