The photon-induced reactivity of liquid ethylene glycol (EG) was investigated in a diamond anvil cell at pressures up to similar to 4 GPa and ambient temperature. The near-UV radiation at gimel = 350 nm was employed to photodissociate EG via the two-photon absorption processes. The reaction evolution was monitored as a function of time and the reaction products were characterized by using in situ FTIR spectroscopy. At low initial loading pressures, the IR spectra show two distinctive sets of profile evaluations indicating sequential photoinduced chemical reactions, which are designated as primary and secondary photochemical processes, respectively. By careful examination of the characteristic IR bands and possible reaction pathways, over ten species as the primary and secondary reaction products were unambiguously identified. Significantly, we found that one of the photodissociation product CO2 forms specific clathrate hydrate structures or clusters that are both time- and pressure-dependent, indicating interesting and unique sequestration behavior of CO2 at high pressures. Quantitative analysis on selective reaction products allows detailed reaction kinetics involving competitive reaction channels to be probed. In particular, the type and quantity of reaction products as well as the kinetics were found highly pressure dependent. Moreover, the pressure variation of the system along the reaction progression allows the interpretation of possible reaction mechanisms of photodissociation of EG under high pressures.