A gas delivery system delivers different process gas compositions to a common supply line at specified times. Multiple reservoirs are fluidly connected to the common supply line with each reservoir having its own charge control valve for controlling connection of the reservoir to the common supply line. Each of the multiple reservoirs has a corresponding mass flow controller and delivery control valve connected to control flow of process gas from within the reservoir to a process module at specified times. The common supply line is operated to fill the multiple reservoirs with different process gas compositions in a time-divided manner. The mass flow controllers and delivery control valves of the multiple reservoirs are operated to deliver one or more process gas compositions to the process module in an accurately timed manner in accordance with a prescribed schedule. The multiple reservoirs are filled as needed to satisfy the prescribed schedule.