The electron-spin g-shifts of the title radicals are calculated using second-order perturbation theory, a Breit-Pauli Hamiltonian, and multireference configuration interaction (MRCI) wave functions. The isotropic [Delta g] values (Delta g = g-g(e)) are -830 for C-2(-)(X(2)Sigma(g)(+)),-6515 for CSi-(X(2)Sigma(+)), and -7595 for Si-2(-)(1(2)Sigma(g)(+)), in ppm, with ppm = 10(-6). For the X(1)A(1) ground states of LiC2, NaC2, and LiSi2, they are -315, -290, and -7770 ppm, respectively. Experimental [Delta g] values in Ar matrices, 3900 ppm for LiC2 and 3535 ppm for NaC2, do not agree with calculated results. However, those predicted for the pi-type 1(2)B(1) excited state of LiC2 and NaC2 (3415 and 3635 ppm, with large parallel components of 9185 ppm for LiC2, and 10 350 ppm for NaC2) are in good agreement with the measured data. Thus, it appears that LiC2 and NaC2 were experimentally prepared, and their ESR spectra measured, in their lowest excited state 1(2)B(1). For the 1(2)B(1) state of LiSi2, [Delta g] is about 9820 ppm. The latter value is mainly determined by a large g(parallel to)-shift of about 28 000 ppm.