Chemically stable solids are in demand for many applications, including pharmaceuticals and organic electronics. Previous efforts in crystals have established the importance of molecular packing in influencing chemical stability by comparing reaction kinetics in different polymorphs. However, efforts to improve chemical stability by modulating molecular packing in amorphous materials have seen much smaller effects. Recently, physical vapor deposition (PVD), a common method to prepare thin films for organic electronics, is reported to create organic glasses with exceptional properties that cannot be accessed by any other method. By vapor-depositing molecules onto a substrate maintained at the temperature below the glass transition temperature (Tg), PVD glasses can demonstrate significantly enhanced thermal stability and increased density relative to traditional liquid-cooled glasses; the optimum substrate temperature usually occurs near 0.85 Tg. The discovery of high-density and high-thermal stability glasses by vapor-deposition provides an opportunity to address questions of how chemical stability can be improved in glasses. The body of this work deals with the characterization of a variety of chemical reactions in organic glasses and establishes the connection between enhanced chemical stability and the distinctive glass packing achieved by vapor-deposition. Films of azobenzenes, photochromic molecules that can undergo trans to cis photoisomerization, were prepared by vapor-deposition at a wide range of substrate temperature. Photostability of vapor-deposited Disperse Orange 37, a push-pull azobenzene with fast cis-trans thermal isomerization, is found to increase by as much as a factor of 50 relative to the liquid-cooled glass. Photostability was determined by measuring density and molecular orientation changes by ellipsometry during irradiation to induce photoisomerization. We further show that the enhanced photostability in vapor-deposited glasses is a general phenomenon by using a non-push-pull azobenzene, 4,4’-diphenyl azobenzene (DPA). By mixing DPA into the glass host of celecoxib, we directly measure populations of trans and cis DPA via UV-Vis spectroscopy and show that the rate of photoisomerization varies as a function of the substrate temperature. Photostability correlates with the density of packing, where the optimum glass is about one order of magnitude more photostable than the liquid-cooled glass. These results show substantially increased photostability of azobenzenes in both neat films and mixtures and provide a molecular explanation for enhanced photostability in glasses. We further investigate the influence of dense glass packing on photodegradation, an important reaction type responsible for degradation in pharmaceuticals and failure in organic electronics. Indomethacin, a pharmaceutical molecule that can undergo photodecarboxylation reaction when irradiated by UV light, was studied as a model system. Photodegradation of indomethacin was assessed through the light-induced mass decrease in glassy thin films caused by the loss of CO2, as characterized by a quartz crystal microbalance (QCM). For the most stable glass, vapor-deposited at 0.85 Tg, the photodegradation rate is about 50% slower than for the liquid-cooled glass when irradiated by a 312 nm UV light. The enhanced stability against degradation correlates with glass density. We speculate that high-density glasses limit the local molecular reconfiguration required for photodecarboxylation. Additionally, to broaden the impact of vapor-deposition on chemical stability, we performed the solid-gas reaction of indomethacin with ammonia. Indomethacin has a carboxylic acid group that can react with ammonia to yield ammonium salt. In this case, chemical reactivity is assessed through the increase in mass induced by the addition of ammonia to glassy thin films, as characterized by a QCM. Vapor-deposited indomethacin with increased density shows slower reaction rates with ammonia relative to the liquid-cooled glass, and the maximum difference in reaction rates is over one order of magnitude. We suggest that the diminished solubility of ammonia in vapor-deposited glasses contributes to their remarkable chemical stability.