A feedstock mixing apparatus for fuel processing systems, and fuel processing and fuel cell systems incorporating the same. A fuel processing system according to the present invention includes one or more fuel processors adapted to produce a product hydrogen stream from a feed stream containing water and a carbon-containing feedstock. The fuel processing system further includes a feedstock delivery system adapted to mix the components of the feed stream at a determined mix ratio and to deliver this feed stream to the fuel processor(s). The fuel processing system may also include one or more fuel cell stacks that are adapted to produce an electric current from the product hydrogen stream produced by the fuel processing system. When the fuel processing system includes at least one fuel cell stack, it may be referred to as a fuel cell system.