We describe an experimental study of diesel-fuel reformation by a plasmatron and diesel-engine exhaust cleaning by means of plasma-chemical pretreatment of fuel. The plasma-chemical reformation of fuel was carried out by a dc arc plasmatron that was designed to increase gas-activation efficiency. Some portion of the fuel was processed by arc discharge and turned into a hydrogen-rich synthesis gas. By using Ar, He, H2, and N2 as cathode protection gases, we discuss the plasma-forming gases and their effects on the reformation process. We analyze the volt-ampere characteristics of the arc discharge in the plasmatron for different gases. The yields of diesel-fuel reformation reached 80%-100% when small quantities of fuel (flow rate of up to about 6 mL/min) were reformed. This technology may be applied to diesel-engine exhaust cleaning by means of the plasma-chemical pretreatment of fuel.