Supersaturated reverse micelles and microemulsions containing either AOT (sodium bis(2-ethylhexylsulfosuccinate)), C12EO4 (poly(oxyethylene-4-dodecyl ether)), or DDAB (didodecyldimethylammonium bromide) surfactants have been used as organized reaction microenvironments for barium sulfate precipitation. Nanoclusters of amorphous barium sulfate, 2-4 nm in size, were formed in association with BaNaAOT reverse micelles. In contrast, highly elongated filaments of crystalline barium sulfate, with lengths up to 100 μm and aspect ratios of 1000, were formed in BaNaAOT microemulsions. The individual filaments were single crystals of barite elongated predominantly along the  axis and in some instances consisted of coaligned nanofilaments, 20-50 nm in width. Similar reactions in C12EO4 reverse micelles and microemulsions afforded barite nanocrystals, 5-7 nm in dimension and tabular-shaped barite crystals, up to 400 nm in size, respectively. Rhombic crystals of barite, approximatety 200 nm in size, were deposited in DDAB microemulsions. No barite filaments were observed in the C12EO4 and DDAB systems. A generalized mechanism for the growth of barite nanofilaments in AOT microemulsions, involving the irreversible fusion, unidirectional exchange, and coalescence of microemulsion droplets, followed by crystallization of an amorphous filamentous BaSO4/surfactant phase, is described.