The interaction of gas phase molecules with other phases bridges two systems—condensed phase and gas phase—that are described by different paradigms of chemical physics. Our research in multiphase systems focuses on understanding how these regimes affect each other and the nature of the intermediary region between the phases. Two areas are emphasized at the CRF: emergence of condensed phase particles from gas phase reactive processes, and reactive interactions between the gas phase and other phases. Chemically controlled gas-to-particle conversion depends on chemical reaction sequences that govern the approach to condensation or particle inception. CRF researchers study these reaction sequences as well as the particles themselves. With multiple implications for energy missions, researchers need to understand the nature of these processes such as soot formation in combustion, aerosol formation in Earth’s atmosphere, particle synthesis processes, or deposit formation in catalytic systems. Reactive interactions between the gas-phase and other phases inform our understanding of catalysis. It is increasingly clear that at atmospheric or greater pressures—conditions under which real catalysis occurs—gas-phase and surface reactions are coupled. CRF scientists apply temporally and spatially resolved probes, developed in gas-phase chemical physics, to map directly the gas phase above reacting surfaces. These measurements can be coupled with advanced operando surface characterization to quantify the interactions between gas phase and surface reactions.