The epitaxial interactions between syndiotactic polypropylene (sPP) and polyethylene (PE) are analyzed on the basis of electron diffraction patterns of composite bilayers. They rest on the near-parallelism of PE chains with short pseudo n-pentane molecular segments of sPP exposed on the (100) plane and inclined by ca. 37 degrees to the helical molecular axis. A related homoepitaxy of sPP, with more limited impact on bulk crystallization, is also described and analyzed on a similar structural basis. It rests on the parallelism of the same n-pentane segments for isochiral helices which face each other across the (110) contact plane and results in chain axes mutually rotated by 74 degrees.