Vacuum ultraviolet photodissociation dynamics of CH3CN and CD3CN has been studied at wavelength near the Lyman-alpha transition (121.6 nm). Two products, CN(B (2) Sigma) and H or D, were observed. The CN(B (2) Sigma-X (2) Sigma) chemiluminescence spectrum was analyzed to determine vibrational and rotational distributions of CN(B). The vibrational distribution was Boltzmann-like and agreed with a statistical distribution. Although the rotation of the upsilon=0-3 states were also characterized by the Boltzmann distribution, the average rotational energy was higher than the statistical energy disposal. The line shape of the Lyman-alpha laser-induced fluorescence of H and D showed a Gaussian Doppler profile, which was ascribed to the statistical translational energy distribution of H and D. Isotope effects were determined for both products. While no isotope effect was observed for energy partition into the vib-rotation of CN(B) and the translation of H and D, relative yields of two products showed an isotope effect, i.e., CN(B) formation was favored for CD3CN over CH3CN and the reverse was true for H(D) atom formation. These results suggest a dissociation mechanism where fast internal conversion occurs after the primary photoexcitation and the vibrationally hot molecule leads to the H(D) atom elimination which competes with C-C bond dissociation.