Studies of non-isothermal crystallization morphologies were carried out in thin films of HDPE ranging in thickness of 15 nm-1.2 mu m, adjacent to either (104) surfaces of calcite crystals or between thin layers of ethylene-octene rubber, both in support of the two preceding investigations of the toughening mechanism of HDPE with rubber and CaCO3 particles. Combined WAXS measurements and AFM imaging of the crystallization forms established that, for films of thickness less than 0.3 mu m, lamellar crystallites preferentially grew ＇edge-on＇, in sheaf-like morphology, on the calcite or rubber interfaces with the (100) crystallographic planes of the lamellae lying parallel to the interfaces. In films thicker than 0.4 mu m the characteristic banded spherulitic morphologies of twisted lamellae became dominant. These observations furnish strong support for the toughening mechanism discussed in the two companion studies (I) and (II).