Two samples of polyisobutylene (PlB 3.9 with ($) over bar M(w) = 3.9 kg mol(-1) and ($) over bar M(w)/($) over bar M(n) = 1.30; and PIB 1.4 with ($) over bar M(w) = 1.40 kg mol(-1) and ($) over bar M(w)/($) over bar M(n) = 1.56) were obtained by means of continuous polymer fractionation. The minimum pressures required to achieve complete miscibility of these polymers with ethylene (Et) in the temperature range from 30 to 70 degrees C are on the order of 200 to 500 bar. The zero shear viscosities of th mixtures are in all cases found to be less than the values calculated from the additivity of eta(0) of the components in terms of volume fractions. The transition of the activation energies and activation volumes from the low values for Et (5 kJ mol(-1) and 6 cm(3) mol(-1)) to the large values for PIB (55 kJ mol(-1) and 80 cm(3) mol(-1)) with increasing polymer content takes places in a pronounced sigmoidal manner.