This thesis describes a diverse set of experiments that probe the interfacial dynamics of complex molecular thin films. Topics include the oxidative and thermal destruction of chemical warfare agents, sticking of small molecules on and in extraterrestrial ices, heterogeneous catalysis in a high-pressure reactor, and oxidation of an important industrial alkene. While the systems of study differ dramatically, all projects focus on uncovering the precise mechanisms of reactivity in condensed films in which diffusion, stable reactive intermediates, and film structure play a crucial role in the observed chemistry. In the oxidative destruction of chemical warfare agent simulants, we find that despite rapid initial reactivity, the build up of a dense product overlayer on the simulant film hinders full destruction of thicker films. Oxygen plays a similarly vital role in the destruction of these compounds under high-temperature, atmospheric pressure conditions. Even initial adsorption can be critically influenced by film structure, as is the case for methane adsorption on water ice. Our work on this system, which has direct relevance to astrophysical environments, demonstrates that high energy methane sticks more readily to porous amorphous ice films than to either crystalline or non-porous amorphous films, likely due to efficient energy accommodation by the pore structure. Even when a reaction isn’t expected, the right high-temperature and high-pressure system can be ripe for heterogenous chemistry. In this case, a supersonic molecular beam nozzle acts as a highly efficient reactor for the production of CO and H2O from a mixture of CO2 and H2. Finally, we find that in the oxidation of condensed propene (an important combustion intermediate species), propene film structure can have a profound effect on the diffusion and subsequent reactivity of oxygen. In addition to highlighting the applicability and effectiveness of time-resolved, surface-sensitive spectroscopic techniques, this work clearly demonstrates the many factors that influence complex reactivity in condensed phases. Product distributions and reaction barriers often differ dramatically due to many-body interactions in the film. Even when barriers are low, contact between reactive species may be significantly hindered by low adsorption probabilities, diffusion constraints, and film morphology.