The relaxation processes and thermal properties of a series of blends of a highly linear high-density polyethylene (HDPE) with several branched high-density, linear low-density (LLDPE) and low-density polyethylenes (LDPE) have been measured. For the sake of comparison, all the samples were subjected to the same thermal treatment, that is, a rapid quenching from the molten state. The mechanical properties have been analyzed, taking into consideration the crystallization behavior of each sample. The existance of cocrystallization provides a bigger change in the dynamic mechanical properties than when segregation takes place. The observed changes in the end-corrected storage modulus supports this conclusion as well as those associated with the α, β, and γ relaxation temperatures.