Heat sealing of films, i.e., formation of a joint between two films by placing them fleetingly between heated platens, was experimentally investigated for a variety of semicrystalline polyolefins, especially various polyethylenes, to determine how sealing temperature affected seal strength measured at room temperature. Seal strength as a function of sealing temperature, SS(T), is closely related to the melting distribution of the polymer determined by DSC measurements, i.e., to the fraction of amorphous phase as a function of temperature, f(a)(T). Seal initiation temperature, the temperature at which a specific, low level of seal strength of polyethylene films is achieved, corresponds to the temperature at which the fraction of amorphous phase equals 77 +/- 3%. At higher temperatures, SS(T) increases approximately as f(a)(T) increases. At the final melting point of the polymer, i.e., when f(a)(T) = 1, seal strength reaches an approximately constant value termed the plateau seal strength. The magnitude of the plateau seal strength is determined by the yield stress of the polymer film. Thus, the heat-sealing curve, SS(T), for a polyethylene can be semiquantitatively predicted from the melting distribution and yield stress of the polymer.