THE rational design of molecular compounds that exhibit spontaneous magnetic ordering might enable one to tailor magnetic properties ties for specific applications in magnetic memory devices(1-4). In such materials synthesized previously(5-17), however, the underlying weak magnetic interactions are incapable of maintaining ordering at ambient temperatures. One remarkable exception is a compound derived from vanadium and tetracyanoethylene(18), but the material is amorphous and fragile, and consequently the molecular interactions responsible for its striking properties are not understood. Here we demonstrate another route to the synthesis of a room-temperature organometallic magnet, in which we combine a hexacyanometalate [M(CN)6](q-) with a Lewis acid L(p+). If L and M are transition-metal ions, then the orbital interactions in the resulting compound can be described by well understood principles(21-24), and it is therefore possible to choose the metals to tune the compound’s magnetic properties-in particular, the magnetic ordering (Curie) temperature T-c(refs 21-26). We have synthesized a room-temperature magnetic material (T-c=315 K) that belongs to the Prussian blue family of compounds(27) (where M is chromium and L is vanadium), demonstrating that transition-metal hexacyano complexes are promising components for the construction of molecule-based high-T-c magnets.