The lattice mismatch in chemically vapor deposited epitaxial &bgr;‐SiC (3C‐SiC) films on 6H‐ and 15R‐SiC (0001) substrates was investigated using a high‐resolution x‐ray diffractometer. The misfit parallel and perpendicular to the growth plane was determined to be (&Dgr;c/c)∥=−9.3×10−4and (&Dgr;a/a)⊥=1.9×10−4for the 3C/6Hsystem, and (&Dgr;c/c)∥=−10.0×10−4and (&Dgr;a/a)⊥=2.3×10−4for the 3C/15Rsystem. Our analysis of the lattice parameters in these three SiC polytypes revealed that the Si‐C pair spacings along thecdirection increased with substrate hexagonality, while the lattice spacings along theadirection decreased with hexagonality. The extent of relaxation was greater in 3Cfilms grown on 6Hsubstrates, a phenomenon attributed to a higher density of double position boundaries. ©1995 American Institute of Physics.