DedicatedtoProfessor Valery Petrosjan,M oscow,ont he occasiono fhis 75th birthday Abstract: The thermalr eactivity of the heteronuclear metal-oxide cluster cations[ XAlO 4 ]C + (X = V, Nb, and Ta )t owards methane has been studied by using mass spectrometry in conjunction with quantum mechanical calculations. Experimentally,ahydrogen-atom transfer (HAT) from methanei sm ediated by all the three oxide clusters at ambient conditions. However,[ VAlO 4 ]C + is unique in that this cluster directly transforms methane into formaldehyde. The absence of this reaction for the Nb and Ta analogues demonstrates as triking doping effect on the chemoselectivity in the conversiono fm ethane. Mechanistic aspects of the two reactions have been elucidated by quantum-chemical calculations. The HATr eactivity can be attributed to the significant spin density localized at the terminal oxygen atom (O t C À )o ft he cluster ions, while the ionic/covalent character of the Lewis acid-base unit [XÀ O b ]p lays ac rucial role for the generation of formaldehyde. Them echanistic insight derived from this combined experimental/computational investigation may provide guidance for am ore rational designo fc atalysts.[a] Dr.