Some universal aspects of interfacial thermodynamics in nearly athermal blends and block copolymers are discussed. While the primary application of the theoretical analysis is to interfaces between polyolefins, the analysis also applies to interfaces between chemically identical but architecturally asymmetric components, such as linear and comb polystyrenes. Specific cases treated are interfaces between linear and short-branched polyolefins and interfaces between linear and comb polymers. For both situations, we derive expressions for the interfacial tension and profile and provide numerical results where appropriate. The implications for the phase diagrams of polyolefin block copolymers are also briefly discussed.