TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Components: (I) comprises a plurality of the first reactant flow paths, and a plurality of corresponding second reactant flow paths. (B) is configured to enable the second reactant to be continuously introduced to (A) along the length of (A) from a portion of the length to the end of the length. (I) further comprises an additional source of the second reactant, where the additional source is configured to enable the second reactant from the additional source to be discretely introduced to a portion of (A) proximate to the beginning of the length of (A). (B) is configured to enable the second reactant to be continuously introduced to a portion of (A) proximate downstream from the beginning of (A). (I) further comprises an additional source of the second reactant, where the additional source is configured to enable the second reactant from the additional source to be discretely introduced to a portion of (A) proximate to the beginning of the length of (A). The catalyst comprises a preferential oxidation catalyst. The second reactant stream comprises air (oxygen). The first reactant stream comprises hydrogen and carbon monoxide; the second reactant comprises oxygen; and the catalyst preferentially oxidizes carbon monoxide over hydrogen. The portions of (A) and (B) are separated by a membrane (oxygen porous). (A) is separated from (B) along the length of (A) from the beginning of the length to the end of the length. (A) is separated from (B) along the length of (A) from a point downstream from the beginning of the length to the end of the length. (A) flows along a channel that channel comprises at least a first side-wall and a second side-wall; and the first side-wall comprises the membrane. The second side-wall comprises a heat exchanger wall. The catalyst is disposed on the first and second side-wall. The membrane comprises yttria stabilized bismuth oxide, silver, ETS-4 zeolite material, solid oxide fuel cell (SOFC) electrolyte material (cerium oxide (CeO2) doped with magnesium (Mg2+), calcium (Ca2+), strontium (Sr2+), ytterbium (Yb3+), yttrium (Y3+), gadolinium (Gd3+), lanthanum (La3+), titanium (Ti4+), zirconium (Zr4+), niobium (Nb5+), scandium (Sc+3), samarium (Sm3+); Zr doped with Ca, Y, yttria-stabilized zirconia; perovskite-type La1-xAxCo1-yByO3-d (where A=Sr, Ba, Ca, B=Fe, Cu, Ni, and d is the non-stoichiometry factor; La0.85Sr0.15Mn_O3); and perovskite-type ABO3, where A=La, Sr, Ca, B=Co and/or Fe; or a polyimide and titanium oxide (TiO2)/polyimide composite). (I) further comprises a fuel cell stack provided with a source of hydrogen gas and a fuel processing system (for providing the hydrogen gas) comprising a primary reactor and the reactor, where the primary reactor is disposed to provide a reactant stream comprising hydrogen and carbon monoxide to the reactor. (I) further comprises: a vehicle body; a fuel cell stack that at least partially provides the vehicle body with motive power; and a fuel processing system. The reactor of (II) defines a plurality of channels, a plurality of first reactant flow paths, and a plurality of corresponding second reactant flow paths. The partition is configured to define a substantially continuous introduction of the second reactant along a portion of the length of the at least one channel downstream from the beginning of the channel. (II) further comprises an additional source of the second reactant, where the second reactant from the additional source is introduced to the first reactant flow path discretely proximate to the channel inlet. The partition comprises a porous membrane. The catalyst comprises a preferential oxidation catalyst. Preferred Method: The preferential oxidation reaction occurs at the preferential oxidation catalyst; the carbon monoxide concentration of the first reactant stream is reduced by the preferential oxidation reaction that converts the carbon monoxide to carbon dioxide; and the efficiency of the preferential oxidation reaction is increased by the continuous introduction of the oxygen from the second reactant stream along at least a portion of the length of the first reactant flow path. The portion of the first reactant flow path and the second reactant flow path are separated by a membrane (oxygen porous). The degree of oxygen porosity of the membrane is such that a suitable amount of oxygen from the second reactant stream is continuously introduced to the first reactant flow path along at least a portion of the length of the channel from the inlet to the outlet. The second reactant flow path is configured such that oxygen from the second reactant stream is continuously introduced to a portion of the first reactant flow path proximate to the end of the length.