A poly(vinyl chloride) (PVC)/poly(methyl methacrylate) (PMMA) (80/20 w/w) polymer blend was studied by mechanical spectroscopy. Two relaxations can be distinguished : in the glassy state, a very large secondary relaxation in the range of 100 K to 325 K which results from the combination of secondary relaxations of PVC and PMMA; and only one main relaxation at 364 K associated to the glass rubber transition. The relaxation spectrum in the range of the beta relaxation has been described by a relaxation time distribution function based upon a Gaussian function and a series-parallel model. The alpha relaxation was studied by means of a theoretical approach for the nonelastic deformation of polymers. We found that the miscibility of this blend appears to be a function of the observation scale : the PVC/PMMA blend is heterogeneous at the scale of molecular movements involved for the beta relaxation process but homogeneous at the scale of the chain segments responsible for the alpha relaxation dynamics.