Recent progress in understanding the phase-dependent reactivity of chlorine dioxide (OCIO) is outlined. Specifically, resonance Raman intensity analysis (RRIA) and time-resolved resonance Raman (TRRR) studies of OCIO photochemistry in solution are presented. The RRIA studies demonstrate that the optically prepared excited-state undergoes significant modification in solution relative to the gas phase. Specifically, the substantial evolution that occurs along the asymmetric-stretch coordinate in the gas phase is restricted in solution. The absence of substantial evolution along the asymmetric-stretch coordinate results in the preservation of ground-state symmetry in the excited state. The role of symmetry in defining the reaction coordinate and the solvent-solute interactions responsible for modification of the excited-state potential energy surface are discussed. TRRR studies performed with pump and probe wavelengths at 390 nm are presented which demonstrate that geminate recombination of the primary photoproducts resulting in the reformation of ground-state OCIO is a central feature of OCIO photochemistry in solution. Time-resolved anti-Stokes experiments are also presented which demonstrate that the OCIO vibrational-relaxation dynamics are solvent dependent. Finally, TRRR studies performed with pump and probe wavelengths of 390 and 260 nm, respectively, are presented. In these studies, 000 production and subsequent decay resulting in Cl production on the subnanosecond time scale is observed. The current picture of 000 photochemistry derived from these studies is discussed, and future directions for study are outlined.