A gaseous fuel catalytic partial oxidation (CPOX) reformer can include a plurality or an array of spaced-apart CPOX reactor units, each reactor unit including an elongate tube having a wall with internal and external surfaces, the wall enclosing an open gaseous flow passageway with at least a portion of the wall having CPOX catalyst disposed therein and/or comprising its structure. The catalyst-containing wall structure and open gaseous flow passageway enclosed thereby define a gaseous phase CPOX reaction zone, the catalyst-containing wall section being gas-permeable to allow gaseous CPOX reaction mixture to diffuse therein and hydrogen-rich product reformate to diffuse therefrom. At least the exterior surface of a CPOX reaction zone of a CPOX reactor unit can include a hydrogen barrier. The gaseous fuel CPOX reformer also can include one or more igniters, and a source of gaseous reformable fuel.