15R polytype of diamond has been synthesized using a specially designed oxy‐acetylene flame system along with 3C diamond and cubic carbon on polycrystalline molybdenum substrates. X‐ray diffraction has been used to detect the 15R phase as the dominant phase in these films. Rapid changes in the substrate temperature during the growth process are expected to be responsible for the growth of these phases. ©1996 American Institute of Physics.