An alternating cyclic (A.C.) method for selectively depositing materials, on the surface of a substrate without depositing the material on an adjacent mask layer. A gas of a reducible compound of the material and a reducing gas, preferably hydrogen, are simultaneously flowed through a reaction chamber to deposit the material on the substrate surface and to a lesser extent on the mask layer. Then, the flow of reducing gas is interrupted to cause the reducible compound gas to etch the material which forms on the mask layer in a disproportionation reaction. The deposition and etch steps are repeated in an alternating cyclic fashion until the requisite thickness is deposited. The process may take place in a single reaction chamber, using only the reducible compound gas and pulsed flow of the reducing gas.