Abstract: Laboratory-hydrolyzed and commercial OH-Al solutions were characterized using kinetics of Al-ferron color development, kinetics of structural OH neutralization with H+, 27Al NMR spectroscopy, and sulfate precipitation. The results showed that the Al13 complexes having the Keggin structure were dominant only in fresh, laboratory-hydrolyzed OH-Al solutions of OH/Al molar ratio = 1.8 and above. These species gradually converted to other polynuclear forms that reacted with ferron slowly, were not detectable by 27Al NMR spectroscopy, and yielded different basic Al sulfates following Na2SO4 addition. These more stable complexes can best be interpreted to have a Al(OH)3-fragrnent structure. In the three commercial aluminum chlorohydrate (ACH) solutions studied, Al13 complexes accounted for a small portion of the total Al present. More than 80% of the Al was present as species that were not detectable with NMR spectroscopy and resembled the slow-reacting complexes in aged, laboratory-hydrolyzed OH-Al solutions. Small portions of the slow-reacting complexes appeared to be submicron particulates that acted as nuclei for gibbsite formation or aggregates of Al13 complexes that dispersed to Al13 upon dilution. Polyaluminum chloride (PA) solution resembled the moderately aged laboratory-hydrolyzed OH-Al solutions.