The use of gas chromatographic analysis of the actual monomer mixture at fixed time intervals to monitor the composition of copolymers in emulsion copolymerization has been described previously. The design has been now improved by the insertion of a dilution cell to avoid flocculation problems in the loop carrying the reaction medium from the reactor to the injection kit of the chromatograph. Then the copolymerization can be monitored up to completion. This system has been applied to the copolymerization of butadiene and acrylonitrile, and constant composition runs have been compared to the batch. Two main differences are observed. (1) Constant composition copolymers show a unique glass transition interval of limited width instead of two or at least one broad temperature interval for the copolymer produced in batch. (2) In the monitored copolymerizations, the production of insoluble gels is delayed and sometimes totally avoided. The production of the gels is related to the formation of 1,2-butadiene units which appear to be preferentially present in long sequences of butadiene units. The cross-linking process involves the consumption of the pendent vinyl groups by copolymerization with the monomers.