In order to determine the influence of composition drift and copolymer microstructure on the mechanical bulk properties of styrene methyl acrylate copolymers, several copolymers were produced by emulsion copolymerizalion. Three different average compositions were used. By perforrming the copolymerizations under batch and semicontinuous conditions with two different monomor addition strategies (starved conditions and optimal addition) it was possible to control composition drift and to produce copolymers with different microstructures (chemical composition distributions). All these copolymers were processed in a way that ensured that the original particle structure was lost before the polymers were tested. It was found that composition drift had an influence on the mechanical properties (Young's modulus, maximum stress, elongation at break). This influence could be understood very well on the basis of present knowledge about structure-mechanical properties relationships. In the case of homogeneous copolymers maximum stress and elongation at break arc dependent on the molecular weight, and only weakly dependent on the chemical composition, and Young's modulus is independent of chemical composition and molecular weight in the range of compositions investigated, as expected. ln the case of heterogeneous copolymers, the influence of copolymer microstructure on Young's modulus, maximum stress and elongation at break is very large. Depending on the extent of control of composition drift during the polymerizations, phase separation was observed in the processed polymers, and the presence of a rubber phase affected the properties profoundly.