The first metastable valence excited states and the first Rydberg states of the MgKr and MgXe molecules have been characterized by resonance two-photon photoionization (R2PI) spectroscopy. The Mg(3s3p(3)P(j)).RG((II0+,0-)-I-3) metastable states, produced by expanding the products of a laser-ablated magnesium rod in Kr/Ar or Xe/Ar gas mixtures into a supersonic expansion, were excited by a dye laser pulse to several vibrational levels of the Mg(3s4s S-3(1)).RG((3) Sigma(+)) Rydberg states, with detection by ionization with a second dye laser pulse. Spectroscopic constants, bond energies, and bond lengths are reported for both states of MgKr and MgXe. The (3) Sigma(+) Rydberg states are much more strongly bound than the lower (II0-)-I-3 valence states, and in fact are essentially as strongly bound as the ground states of the analogous MgRG(+) ions, characterized previously in the same apparatus. This clearly indicates that the RG atoms can readily penetrate the diffuse Mg(4s) Rydberg electron cloud. The interesting and unusual spin-orbit and "spin-spin" effects observed are attributed to mixing of some RG character into wave functions of predominantly Mg* excited state character. Bonding and spin-orbit interactions in the MgAr, MgKr, and MgXe first triplet metastable and Rydberg states are discussed. (C) 1997 American Institute of Physics.