Electron spin resonance (ESR) studies are reported for the first time on the various isotopomers of the CH3Cd radical isolated in neon and argon matrices. The radicals were generated in neon matrices by the reaction of laser-ablated cadmium metal and various methyl precursors, and in argon matrices by x-irradiation. The neon matrix values measured were g(perpendicular to)=1.9491(1), A(perpendicular to)(H)=17.0(1) MHz, A(perpendicular to)(D)=2.6(1) MHz, A(perpendicular to)(C-13)=163(3) MHz, and A(perpendicular to)(Cd-111)=-3083(3) MHz, and estimates were derived for A(parallel to)(C-13)=230(50) MHz and A(parallel to)(Cd-111)=-3486(5) MHz. The argon matrix values measured were g(perpendicular to)=1.952(1), A(perpendicular to)(H)=16(1) MHz and A(perpendicular to)(Cd-111)=3301(3) MHz, and an estimate was derived for A(parallel to)(Cd-111)=-3704(5) MHz. The ESR experimental A(dip)(Cd-111) values for the neon and the argon matrices agree with the reported gas-phase value [J. Chem. Phys. 101, 6396 (1994)]. The matrix ESR A(iso)(Cd-111) values show small shifts compared with the gas-phase results (5% greater for the neon matrix and 12% greater for the argon matrix). At 4.3 K in the neon matrices, additional ESR lines assigned to tunneling phenomena were observed. The radical geometry obtained from ab initio calculations was consistent with that reported from the various experimental results. Multireference single and double excitation configuration interaction (MRSDCI) calculations of the hyperfine interactions gave values that were consistently below the experimental values of A(iso) and A(dip) for Cd-111, H, and C-13. MRSDCI calculations for the CdH radical showed an analogous trend.