An apparatus for depositing materials on a substrate includes a manifold having a plurality of inlet valves located equidistantly from the manifold outlet. Preferably the inlet valves are mounted in radial configuration to minimize any "dead space" between the valves and the manifold outlet. The manifold connects a plurality of gas sources through its inlets to a process chamber at its outlet. The valves within the manifold switch continuous reactive gas flows from the sources back and forth between the process chamber and a vent chamber during the deposition process. A purging gas flow is also provided at each valve to purge the "dead space" within the manifold of reactive gases that can linger once the reactive gas flow has been switched to the vent. The method employed by the apparatus maintains a predetermined constant gas flow through the process chamber to produce uniform deposition on the substrate. Reactive and nonreactive gas flows are injected into the process chamber at a rate sufficient to maintain the predetermined flow. After each deposition step, reactive gas flows are switched into and out of the process chamber and the nonreactive gas flows are adjusted in response to maintain a constant total flow. The vent chamber is maintained at substantially equal pressure to the process chamber to minimize fluctuations in reactive gas flows as the flows are switched between chambers.