Incorporation of water into sulfuric acid-bisulfate anionic clusters was more favorable compared to incorporation of sulfuric acid into protonated water clusters. This laboratory study supports field studies that showed anionic nucleation of sulfuric acid-water system as more favorable than cationic or neutral nucleation. In fact, neutral nucleation was found to be slower than ion-induced nucleation under tropospheric conditions.( ) Extensive hydrogen bonding and proton transfer between ammonia and sulfuric acid results in fully ionized cluster components (NH 4 + (NH 3 ) n (H 2 SO 4 ) s ). The corresponding structures were 50 kcal mol -1 more stable than clusters composed entirely of neutral ammonia and sulfuric acid, with clusters where n ≥ s were found to be particularly stable. Thus, sulfuric acid is stabilized to a greater extent by ammonia and water, making sulfuric acid-ammonia nucleation more spontaneous in the free troposphere.