Microstructure of and mechanical properties of a series of symmetric EPE (E = poly(ethylene-co-1-butene) and P = poly(ethylene-alt-propylene)) triblock copolymers with M n = 27-372 kg/mol have been investigated in the solid state. Microphase separation of EPE triblock copolymers originates from either (i) chemical incompatibility-induced microphase separation in the melt before crystallization at high overall molecular weights or (ii) crystallization-induced microphase separation from a homogeneous melt at low overall molecular weights. Small-angle X-ray scattering demonstrates that these EPEs (f E ≈ f F ≈ 0.5) adopt a lamellar morphology over the range of molecular weights studied. The observed interlamellar domain spacing (d) scales with the molecular weight as d ∼ N 0 β, where N 0 is the degree of polymerization and β= 0.22 for crystallization-induced segregation and β= 0.63 for microphase-separated melts. The mode of microphase separation strongly affects the chain folding microstructure of the polyethylene crystallites within the lamellar structure. The consequences of these observations on the ultimate tensile strengths of these materials are discussed.