The stretch frequencies of the C-H and C-D modes in the (V-Si + H) complex of 6H SiC are given out to the third harmonic. A comparison is made with the frequencies of C-H and C-D modes in 15R SiC out to the second harmonic. A Morse Potential is found to account well for both the anharmonicity and the isotope effect found for the C-H and C-D centers. A first principles calculation for 3C SiC accounts well for the ground state and higher harmonic frequencies found in these experiments as well as the anharmonicity parameter.