Quantum chemical calculations at the Hartree-Fock, B3LYP, MP2, and CCSD(T) levels of theory were performed in order to study the mechanism of Ta2O5 chemical vapor deposition (CVD) from TaCl5 and H2O. The geometries and vibrational frequencies of reactants, products and transition states of the reactions, which are used for modeling the initial steps of the CVD process were calculated using effective core potentials for Ta and Cl atoms. The reaction between TaCl5 and H2O proceeds via formation of a strongly bonded six-coordinated tantalum complex (25 kcal/mol at CCSD(T)//B3LYP). The correlated methods show the transition state energy to be close to the energy of the initial reagents. Formation of TaOCl3, in the unimolecular decomposition of TaCl4OH and TaCl3(OH)(2), has similar barriers of similar to 20 kcal/mol. The catalytic effect of an assisting water molecule opens a barrierless channel for TaOCl3 formation in the gas phase.