Physical and catalytic properties of hydrogen molybdenum bronze, H1.55MoO3 varied markedly with H-2 reduction. The surface area was enlarged from 10 m(2)/g to 250 m(2)/g by H-2 reduction at 673-723 K. H1.55MoO3 became an active and selective catalyst for pentane isomerization after reduction. H-2-reduced H1.55MoO3 exhibited a comparable isomerization activity to H-2-reduced Pt/MoO3.