The spin-forbidden radiative decay process CH(a (4) Sigma(-))-->CH(X (2) Pi) is studied using the full Breit-Pauli spin-orbit Hamiltonian and multireference configuration interaction wave functions comprised of 250 000-350 000 configuration state functions. The radiative lifetime of the (a (4) Sigma(-),v,N,F-i)-->X (2) Pi transitions were determined in a Hund’s case (b) approximation. The pure radiative lifetimes are found to be quite long, on average being 12, 10, and 8 s for v=0,1,2, respectively.