The linear viscoelastic behavior in the melt of PMMA/PS blends and various blends of PMMA containing rubbery latex particles has been characterized by dynamic shear rheometry. For the rubber‐toughened PMMA samples, the influence of rubber content, structure of the latex particles, and aggregation of the particles in the PMMA matrix have been investigated. Morphologies of the dispersed type or of the aggregated type were produced by performing the blending in the melt or in solution. The data forG’andG‘have been analyzed in the frame of a rheological emulsion model which is found to account for the behavior of the PMMA/PS blends and of rubber‐toughened PMMA at low rubber content. At high rubber concentrations the model does not predict the secondary plateau inG’which arises at low frequencies for these systems. Therefore, this plateau cannot be attributed to the deformability of dispersed inclusions as in PMMA/PS blends, but is shown to depend on the extent of aggregation of the dispersed particles, and to be most important in a well dispersed morphology where the particles form a network‐type structure.