Powder samples of hydrogen molybdenum bronzes (HxMoO3) were effectively stabilized by the addition of 30 mass% polyethylene (PE) powder, pressing and a following thermal treatment, The composite material does not degrade in 0.05 mol l(-1) H2SO4 but shows 1-3 mu m wide cracks in the surface and the formation and growth of small hydrated bronze crystals (1-10 mu m) during electrochemical cycling. An electrochemical dissolution/precipitation mechanism is presented and discussed.