The resistance to slow crack growth (SCG) was measured in binary blends of high density polyethylene (HDPE) and 5-10% concentrations of model ethylene-butene random copolymers by measuring the time to failure (t(f)) under a constant stress intensity. An increase of ti with the addition of the copolymer if the copolymer could crystallize and the increase was greater the higher branch density. The copolymer with 117 branches/1000C could not crystallize and therefore its blend had a ti that was less than that of the HDPE. The fracture energies of the blends as determined by their resistance to SCG were compared with the energy by rapid fracture, J(c), as previously measured by Rhee and Crist. It is concluded that SCG is more sensitive to variations in the microstructure than is rapid fracture and that the differences in SCG behavior can be qualitatively explained in terms of the differences in microstructure of the blends.