Improved feedstocks for hydrogen-producing fuel processing and fuel cell systems, and fuel processing and fuel cell systems incorporating the same. The fuel processing systems include a fuel processor adapted to produce a product hydrogen stream from at least one feed stream containing at least a carbon-containing feedstock and an aversive agent. The fuel processing systems may also include a fuel cell stack adapted to produce an electric current from the product hydrogen stream. The feedstock is at least substantially formed of a hydrocarbon or alcohol. In some instances the feedstock includes methanol and the aversive agent is delivered to a hydrogen-producing region of the fuel processor with the carbon containing feedstock. The aversive agent can include at least one odorant, flavorants regurgitant, and/or dye. In other instances, the feed stream includes water or the feed stream is delivered to a fuel stack that produces and electric current therefrom.