Kinetic information on particle growth is obtained for the ab initio emulsion polymerization of butadiene. It is shown that the decrease in particle number by coagulation (induced by the high ionic strength) hardly affects polymerization rate, since the average number of radicals per particle (n̄) increases with particle size. From a rate analysis of intervals II and III it follows that the system is "zero-one" (n̄ ≤ 0.5), i.e., termination is not rate-determining. Zero-one kinetics, in combination with a low initiator efficiency, explains the small effect of initiator concentration on polymerization rate. The radical loss mechanism responsible for the zero-one kinetics could not be established unambiguously, but chain transfer/desorption processes involving polybutadiene, thiol, surfactant, and the Diels-Alder dimer 4-vinyl-1-cyclohexene were all refuted on experimental grounds. Desorption of monomeric species seems a reasonable explanation, given the rather low estimate for the propagation rate coefficient.