Assuming that the viscosimetric behaviour at low frequencies is mainly determined by the behaviour of the continuous phase, it becomes possible to explain the behaviour of PS/PMMA blends from a limited number of hypotheses. The main hypothesis is that the PMMA layer next to the interface gelifies, while the PS layer next to the interface slips under the influence of a hydrodynamical field. The explanation concurs with all the experimental data which we obtained, regardless of the concentration of PS or PMMA. Moreover the effect of adding a block copolymer PS-b-PMMA can be explained.