Terpolymers from 1,3-butadiene, styrene, and vinylpyridine are important as rubbers, and, in latex form, as components of rubber-to-fabric adhesives. Three monomer mixtures having the approximate weight composition butadiene/styrene/2-methyl-5-vinylpyridine 70/15/15, 25/48/27, and 38.5/49/12.5 have been emulsion-polymerized at 30°C., and the terpolymer composition as a function of the conversion has been studied. The six monomer reactivity ratios have been determined by binary copolymerizations at 30°C. and checked by low conversion bulk terpolymerizations. Average and instantaneous composition–conversion curves have been calculated, on the assumption of a constant reaction volume, by numerical integration of the terpolymerization equations on electronic computer. Details, of general applicability, for calculations are given. The experimental results of the emulsion terpolymerizations fit satisfactorily the calculated curves over a wide range of conversions.