One-phase microemulsion regions at 25 and 60 A degrees C and the polymerization at 60 A degrees C in o/w microemulsion formed by 1,3-butadiene, water, and a mixture of the surfactants, dodecyltrimethylammonium bromide, and didodecyldimethylamonium bromide (3/1 w/w), are reported. The polybutadienes obtained here have similar characteristics to those of their homologous obtained by emulsion polymerization, with the only difference that the average particle size of the former (25-30 nm) was smaller by an order of magnitude. The obtained polymer had high average molecular weights, but the gel content at high conversion is lower than the value for a conventional emulsion polymerization. The DSC measurements showed that the polymer has a single glass transition temperature at -72.5 +/- A 1.5 A degrees C.