The water-gas shift (WGS) reaction is an industrially important source of pure hydrogen (H-2) at the expense of carbon monoxide and water(1,2.) This reaction is of interest for fuel-cell applications, but requires WGS catalysts that are durable and highly active at low temperatures(3). Here we demonstrate that the structure (Pt1Ptn)/alpha-MoC, where isolated platinum atoms (Pt-1) and subnanometre platinum clusters (Pt-n) are stabilized on alpha-molybdenum carbide (alpha-MoC), catalyses the WGS reaction even at 313 kelvin, with a hydrogen-production pathway involving direct carbon monoxide dissociation identified. We find that it is critical to crowd the alpha-MoC surface with Pt-1 and Pt-n species, which prevents oxidation of the support that would cause catalyst deactivation, as seen with gold/alpha-MoC (ref. (4)), and gives our system high stability and a high metal-normalized turnover number of 4,300,000 moles of hydrogen per mole of platinum. We anticipate that the strategy demonstrated here will be pivotal for the design of highly active and stable catalysts for effective activation of important molecules such as water and carbon monoxide for energy production.