Absorption, fluorescence-excitation, and fluorescence spectra as well as fluorescence yields and decay times are recorded for Hg2 molecules in argon matrices pure and doped with small quantities of water or ammonia. All excited electronic states of Hg2 decay rapidly to the lowest metastable A0g+/- state inducing a long-lived (tau congruent-to 200 mus) luminescence with quantum yield close to unity. The gas-to-matrix shifts and the effects of Hg-NH3 and Hg-H2O interactions are discussed in relation to the spectroscopic properties of free, jet-cooled Hg2Arn and Hg2(X)n (where X = H2O or NH3) [Chem. Phys. Lett. 170, 153 (1990); Z. Phys. D 20, 73 (1991); J. Chem. Phys. 98, 5281 (1993); Laser Chem. (in press); S. Matrenchard-Barra thesis, Orsay, 1993].