Multiple glass transitions in emulsion polymerized butadiene—acrylonitrile copolymer at leass than 36% acrylonitrile by weight were studied using the Differential Scanning Calorimeter. The appearance of two rather than one transition was shown to be related to the drift in composition with degree of monomer conversion, when all monomer was introduced initially. Material responsible for either glass transition in 29% ACN copolymer was concentrated by phase separation in solution. Phase separation in the butadiene—acrylonitrile copolymer system when the width of the distribution of compositions is above a critical value is shown to be predicted by Scott's thermodynamic treatment of multi-component copolymer mixtures. The partitioning of species of different compositions between phases was calculated for a model distribution of polymeric compositions.