The dielectric function was determined in the alpha beta splitting region of a series of amorphous random copolymers of n-propylmethacrylate (nPrMA) with n-pentylmethacrylate (nPeMA). The intensity of the alpha process decreases with increasing temperature in all copolymers investigated. The onset temperature of the alpha process intensity decreases with increasing nPeMA content. The splitting behaviour continuously changes with composition. These results were confirmed by dynamic shear modulus measurements in the alpha beta splitting region. The shift of the PnBMA splitting region due to addition of plasticizer.