Nanosized insoluble metal sulfate aggregates, predominantly CaSO4, have been prepared in a variety of water-in-oil (w/o) microemulsions stabilized by either nonionic or ionic surfactants. Particles were visualized by transmission electron microscopy (TEM) and the identities of the aggregates confirmed by energy-dispersive X-ray analysis (EDXA). BaSO4 prepared in n-heptane microemulsions stabilized by the sodium salt of Aerosol-OT (AOT) appeared as slightly irregular aggregates, 8-50 nm in diameter. In contrast, BaSO4 in n-heptane microemulsions stabilized by ammonium diethylhexyl phosphate (NH4-DEHP) existed in the form of submicron-sized "flocs" comprising nanospheres 5-7 nm in diameter. BaSO4 synthesis in cyclohexane microemulsions stabilized by tetraethylene glycol monododecyl ether (C12E4), produced discrete essentially monodisperse nanospheres 8-10 nm in diameter. A wider variety of morphologies were encountered in the synthesis of CaSO4 which produced nanospheres, ellipsoids, rods, nanohairs, nanowires, and nanobundles. The greatest structural diversity was obtained in systems stabilized by C12E4 where product morphology was sensitive to the mole ratio of water to surfactant (omega(0)) in the reaction medium, the overall water content, surfactant concentration, reactant concentration, and incubation time. The growth of CaSO4 nanowires was monitored as a function of time and was fastest at high overall reactant concentrations. Nanowires and nanobundles were often observed to span completely the individual sections of the copper grid used in TEM measurements, indicating lateral growth potential on the order of hundreds of microns. In contrast, CaSO4 formed in microemulsion systems stabilized by AOT in dodecane yielded only nanospheres whose size was largely independent of composition and reaction conditions.