Catalytic aqueous polymerization of butadiene by the cobalt catalyst system [Co(C8H13)(C4H6)]/CS2 affords colloidally stable latices of highly crystalline syndiotactic 1,2-polybutadiene (T-m = 190 degrees C, Delta H-c = 60 J/g, M-w similar to 10(5) g/mol, 97% 1,2, 3% 1,4-cis). Catalyst modification with reagents containing electrophilic carbon atoms (e.g., aromatic formamides) strongly decreases polymer crystallinity (T-m = 60-100 degrees C, Delta H-c = 11 J/g, M-w similar to 8 x 10(4) g/mol for 85% 1,2). Latices of typically 150-200 nm particles are prepared. During polymerization affording low crystalline polymers the number of particles stays constant. Microstructures were analyzed by C-13 NMR of hydrogenated polymers. In all polymers the 1,4-units are largely isolated. Crystallinity decrease results from an increase of 1,4-insertion frequency and the resulting decrease of 1,2-block length. Considerations on the nature of the active species are discussed. Copolymerization of butadiene with isoprene by [CO(C8H13)(C4H6)]/CS2 affords low crystalline copolymers; styrene is not incorporated as a comonomer.