The main nucleating vapor in the atmosphere is thought to be sulfuric acid (H ₂ SO ₄ ), stabilized by ammonia (NH ₃ ). However, in marine and polar regions, NH ₃ is generally low, and H ₂ SO ₄ is frequently found together with iodine oxoacids [HIO _x , i.e., iodic acid (HIO ₃ ) and iodous acid (HIO ₂ )]. In experiments performed with the CERN CLOUD (Cosmics Leaving OUtdoor Droplets) chamber, we investigated the interplay of H ₂ SO ₄ and HIO _x during atmospheric particle nucleation. We found that HIO _x greatly enhances H ₂ SO ₄ (-NH ₃ ) nucleation through two different interactions. First, HIO ₃ strongly binds with H ₂ SO ₄ in charged clusters so they drive particle nucleation synergistically. Second, HIO ₂ substitutes for NH ₃ , forming strongly bound H ₂ SO ₄ -HIO ₂ acid-base pairs in molecular clusters. Global observations imply that HIO _x is enhancing H ₂ SO ₄ (-NH ₃ ) nucleation rates 10- to 10,000-fold in marine and polar regions.