Tungsten (W) films were deposited on Si(100) from tungsten hexacarbonyl, TW(CO),I, by low-pressure chemical vapor deposition (CVD) in an ultra-high vacuum (UHV)-compatible reactor. The chemical purity, resistivity, crystallographic phase, and morphology of the deposited films depend markedly on the substrate temperature. Films deposited at 375 degrees C contain approximately 80 at.% tungsten, 15 at.% carbon and 5 at.% oxygen. These films are polycrystalline P-W with a strong (211) orientation and resistivities of >1000 mu Omega cm. Vacuum annealing at 900 degrees C converts the metastable beta-W to polycrystalline OL-W, With a resistivity of approximately 19 mu Omega cm. The resultant alpha-W films are porous, with small randomly oriented grains and nanoscale ((100 nm) voids. Films deposited at 540 degrees C are highpurity (>95 at.%) polycrystalline a-W, with low resistivities (18-23 mu Omega cm) and a tendency towards a (100) orientation. Vacuum annealing at 900 degrees C reduces the resistivity to approximately 10 mu Omega cm, and results in a columnar morphology with a very strong (100) orientation.