An investigation of the area of relative altitudes of co-planar circular orbits for which a bi-elliptical transfer is more economical than the classical Hohmann transfer. It is found that for transfer between orbits apart by a radii-ratio larger than approximately 15·58, the Hohmann transfer is less economic than any bi-elliptical transfer with an intermediate point of altitude larger than that of the larger terminal orbit. For radii-ratios less than 15·58 but greater than approximately 11·94, the bielliptical transfer is more economical if the intermediate point is placed at a sufficiently high altitude. This altitude is infinite at 11·94 and approaches the altitude of the larger terminal orbit at 15·58.