Atomically ordered intermetallic nanoparticles are promising for catalytic applications but are difficult to produce because the high-temperature annealing required for atom ordering inevitably accelerates metal sintering that leads to larger crystallites. We prepared platinum intermetallics with an average particle size of <5 nanometers on porous sulfur-doped carbon supports, on which the strong interaction between platinum and sulfur suppresses metal sintering up to 1000°C. We synthesized intermetallic libraries of small nanoparticles consisting of 46 combinations of platinum with 16 other metal elements and used them to study the dependence of electrocatalytic oxygen-reduction reaction activity on alloy composition and platinum skin strain. The intermetallic libraries are highly mass efficient in proton-exchange-membrane fuel cells and could achieve high activities of 1.3 to 1.8 amperes per milligram of platinum at 0.9 volts.