Abstract: A proportioning pumping system that injects an injection fluid into a pressurized conduit flowing with a working fluid at a constant proportion of injection fluid to working fluid regardless of changes in flow rate or pressure within the working fluid conduit. Injection fluid is pumped continuously, by the working fluid, from a non-pressurized tank. The system includes two or more vessels, each divided into two chambers by a diaphragm or bladder. Valving and passages simultaneously fill one vessel with working fluid and pump injection fluid while filling the other vessel with injection fluid and draining working fluid. A first pressure differential creating device in the working fluid conduit draws injection fluid into the working fluid conduit. A second pressure differential creating device determines the proportion of working fluid and injection fluid to be combined.