The frequency shifts from gas-phase values of the vibrational and rotational states of H2, D2, and HD mols. matrix isolated in solid Ar, Kr, and Xe were calcd. at zero temp. and at pressures 0 ? P ? 570 kbar. Also calcd. were the impurity local mode frequencies vs. pressure. The pure vibrational and rotational-vibrational transitions are strongly red shifted in the solid P = 0, compared to the gas-phase values. This shift is greatest for Xe and smallest for Ar host lattices. The agreement with Raman scattering exptl. data is good. The pure rotational transitions also show a small red shift at P = 0 in the solid, and are in generally good agreement with the measurements of H. Jodl and K. Bier (to be published) but not with those of G. Prochaska and L. Andrews (1977) who, except for D2 (Ar) measure small blueshifts. With increasing pressure, all transitions are strongly blueshifted with respect to P = 0 solid frequencies, except at relatively low pressures where most of the modes of H2, D2, and HD in Xe and Kr are initially red shifted. The calcd. local mode frequencies of the impurity mols. in the solid at P = 0 are in good agreement with expt., and are strongly blueshifted with increasing pressure. [on SciFinder (R)]