The Mg(3s3d pi D-3(J)). Ar((3) Pi), Mg(3s3d delta D-3(J)). Ar((3) Delta), and Mg(3s4p pi P-3(J)). Ar((3) Pi) states have been characterized via resonance enhanced two-photon ionization (R2PI) spectroscopy of transitions from the long-lived Mg(3s3p pi P-3(J)). Ar((3) Pi(0+,0-)) metastable states of the MgAr van der Waals molecule. Because the outer excited Mg(3d) and Mg(4p) orbitals are (similarly) quite diffuse and the Ar atom can approach along the nodal axes of each of the aligned orbitals, minimizing repulsion, the Mg(3s3d delta D-3(J)). Ar((3) Delta) and Mg(3s4p pi P-3(J)). Ar((3) Pi) states are both very strongly bound (D-0=1140+/-40, 1250+/-60 cm(-1), respectively), approaching the bond energy of the MgAr+ "core" ion (for which D-0=1240+/-40 cm(-1)). In contrast, the Mg(3s3d pi D-3(J)). Ar state is more weakly bound (D-0=290+/-40 cm(-1)), although it has a greater bond strength than the Mg(3s3p pi P-3(J)). Ar((3) Pi(0-)) lower state (for which D-0 is 160+/-40 cm(-1)). The effective spin-orbit constant of the Mg(3s3d pi D-3(J)). Ar((3) Pi) state is much larger than expected from the Mg(3s3d D-3(J)) multiplet splittings, and also increases in magnitude as upsilon’ decreases; possible reasons for this are discussed. It is suggested that the predissociation of the Mg(3s3d pi D-3(J)). Ar((3) Pi(0+)) state [the Mg(3s3d pi D-3(J)). Ar((3) Pi(0-)) state does not predissociate] is caused by a curve-crossing with the + repulsive Mg(3s3d sigma D-1(2)). Ar((1) Sigma(0+)(+)) state, which lies at a lower asymptotic energy because of substantial mixing of Mg(3p3p D-1(2)) character into its wave function.