The first of a new class of strongly bound, double excited valence states of neutral van der Waals molecules, Mg(3p pi,3p pi (3)p(J)) . Ar((3) Sigma(-)), has been characterized by R2PI (resonance two-photon ionization) spectroscopy via excitation of metastable Mg(3s3p pi P-3(J)) . Ar((3) Pi(0+,0-)) states in a supersonic jet. The MgAr(3 Sigma(-)) state has a bond energy of similar to 2900 cm(-1), more than twice that of the MgAr+ ion.