The resistance to slow crack growth and to internal over-pressure of pipes based on a commercial polyethylene gas pipe grade (BPE; 0.6 mol% butyl branches and (M) over bar(w) = 197,000 g/mol) blended with both an injection molding grade Linear polyethylene (LPE; (M) over bar(w) = 42,000 g/mol) and 0.5 +/- 0.1 mm glass spheres was studied, The blends and the pure polymer components were characterized by density measurements, differential scanning calorimetry, small-angle X-ray scattering, and transmission electron microscopy. The fracture properties of extruded pipes mere measured using hydrostatic pressure testing and notched uniaxial testing on samples cut from the pipe wall. The polymer components were macroscopically uniformly blended and differential scanning calorimetry indicated the occurrence of partial co-crystallization between the branched and linear components. Transmission electron microscopy showed molar mass segregation on a 100 nm level. Notched uniaxial testing showed that the slow crack growth resistance of pure BPE was considerably higher than that of the LPE/BPE blend with 30% LPE. Fractography indicated that the fracture-initiating particles were larger in pipes failing after shorter period of time in the hydrostatic pressure testing. The lifetimes of hydrostatic pressure tested pipes based on the BPE grade containing glass spheres were similar to these of pipes based on the LPE/BPE blend with 30% LPE.