Results of gas phase reactivity studies on group six transition metal suboxide clusters, Mo(3)O(y) (-), Mo(2)WO(y) (-), MoW(2)O(y) (-), and W(3)O(y) (-) (Mo((3-x))W(x)O(y) (-), x=0-3; y=ca. 3-9) with both D(2)O and CO(2) are reported. Sequential oxidation for the more reduced species, Mo((3-x))W(x)O(y) (-)+D(2)O/CO(2)-->Mo((3-x))W(x)O(y+1) (-)+D(2)/CO, and dissociative addition for certain species, Mo((3-x))W(x)O(y) (-)+D(2)O/CO(2)-->Mo((3-x))W(x)O(y+1)D(2) (-)/Mo((3-x))W(x)O(y+1)CO(-), is evident in the product distributions observed in mass spectrometric measurements. Reactions with D(2)O proceed at a rate that is on the order of 10(2) higher than for CO(2). The pattern of reaction products reveals composition-dependent chemical properties of these group six unary and binary clusters. At the core of this variation is the difference in Mo-O and W-O bond energies, the latter of which is significantly higher. This results in a larger thermodynamic drive to higher oxidation states in clusters with more tungsten atoms. However, addition products for more oxidized W-rich clusters are not observed, while they are observed for the more Mo-rich clusters. This is attributed to the following: In the higher oxides (e.g., y=8), addition reactions require distortion of local metal-oxygen bonding, and will necessarily have higher activation barriers for W-O bonds, since the vibrational potentials will be narrower. The binary (x=1,2) clusters generally show sequential oxidation to higher values of y. This again is attributed to higher W-O bond energy, the result being that stable binary structures have W atoms in higher oxidation states, and Mo centers both in more reduced states and sterically unhindered. The reduced Mo center provides a locus of higher reactivity. An unusual result that is not readily explained is the chemically inert behavior of Mo(3)O(6) (-).