NOVELTY - Vaporizing liquids to produce a mixed vapor stream (comprising water vapor, gas and a fuel vapor) comprises passing a liquid water stream through a first nozzle at high linear velocity to give a high velocity water stream; passing a liquid fuel stream through a second nozzle at high linear velocity to give a high velocity fuel stream; and contacting the water stream, in the presence of a gas, and the high velocity fuel stream with a water and fuel vaporizing bed; and an apparatus to produce the mixed vapor stream comprises the first and second nozzles and at least one vaporizing bed.    USE - The mixed vapor stream is used to operate reforming catalysts to produce a mixture of hydrogen and carbon oxides, e.g. in laboratory testing of reforming catalyst performance or in a vehicle-mounted catalytic reformer for producing hydrogen for use with fuel cells.    ADVANTAGE - The method is an improved method that separately vaporizes the liquid water and the liquid fuel. The apparatus comprises separate vaporizers each having a single heat source, thus the vaporizers are readily operated at different temperatures. The ratio of the steam to carbon and the ratio of oxygen to carbon in the mixed vapor stream can be changed rapidly or kept constant when changing space velocities, which is beneficial when the mixed vapor stream is used to operate reforming catalysts to produce a mixture of hydrogen and carbon oxides, e.g. in laboratory testing of reforming catalyst performance or in a vehicle mounted catalytic reformer for producing hydrogen for use with fuel cells. The composition of the mixed vapor stream comprising water and fuel vapors is maintained stable and yet also changed rapidly to another composition or space velocity, which property is important for vehicle-mounted fuel cell applications where the composition of the vapor feed to the reformer needs to be changed rapidly in response to downstream conditions, e.g. to optimize vehicle energy efficiency and increase catalyst life and, in particular, to avoid low or high (local) concentrations of fuel vapor or steam in the feed to the reformer which can cause coking of the reformer catalyst.