Termination rate coefficients for poly(methyl methacrylate) (PMMA) propagating radicals have been determined from ESR measurements of the propagating free radical concentration during batch emulsion polymerization. At conversions greater than or equal to similar to 99%, the termination is clearly second order in radical concentration. At conversions of similar to 90-96%, a rapid termination occurs in larger latex particles but not in smaller ones. If we assume that this termination is bimolecular, then the termination rate coefficient is not constant. The rapid termination rate coefficients are up to similar to 2 orders of magnitude higher than the very high conversion termination coefficients. Comparison of the termination rate coefficients determined here, those determined by other workers, and predictions from chain-length-dependent termination theory suggest that (1) the high conversion termination is residual or reaction diffusion-chains ends do not move freely but approach each other by adding monomer, (2) the chain-length-dependent termination theory probably overestimates the residual termination coefficient somewhat, and (3) some short-long termination probably occurs early in the rapid termination period. We hypothesize that the rapid termination occurs in the larger particles because of radical distribution inhomogeneity.