A process is described for the preparation of viscosity index improver (V.I.I.) additives which includes treatment under high shear conditions of a composition comprising (i) one or more EP(D)M polymers and (ii) one or more polyvinylarene/hydrogenated conjugated diene/polyvinylarene block copolymers, the weight ratio (i)/(ii) ranging from 98:2 to 80:20, preferably from 97:3 to 90:10, the above process being carried out at a temperature ranging from 150.degree. C. to 400.degree. C., preferably from 180.degree. C. to 320.degree. C.