Nitrogen has a complex phase diagram that has presented a formidable challenge to theoretical models. In this study, infrared (IR) spectroscopy has been performed on solid nitrogen at low temperatures and pressures up to 6 GPa. In the epsilon phase, two IR absorption peaks were observed that correspond to N-N stretch modes. The presence of two IR-active N-2 vibrons is consistent with a tetragonal structure (P4(3)2(1)2) and is inconsistent with an earlier proposed rhombohedral (R (3) over barC) structure. The matrix isolation of CO2 molecules in solid N-2 led to the observation of transverse (v(2)) vibrational modes, which showed a splitting consistent with the structures of the different N-2 phases. The v(3) mode of (CO2)-C-12 and (CO2)-C-13 molecules was measured as a function of pressure, as well as combination modes. All the vibrational-mode frequencies shifted discontinuously at the critical pressures for N-2 phase transitions. For CO2 Concentrations of 0.1%, phase separation is present at low pressures, but is suppressed for pressures greater than 1 GPa. In summary, these studies have contributed to the knowledge of the N-2 phase diagram and the nature of guest-host interactions in molecular systems.