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Monolayer Molybdenum disulfide (MoS₂) is a two-dimensional transition metal dichalcogenide whose structure is highly sensitive to defect engineering. In our work, we present a quantitative X-ray photoelectron spectroscopy (XPS) investigation of ion beam-enhanced reactivity in monolayer MoS₂, using air oxidation kinetics as a probe of defect-mediated chemical activity. Controlled ion irradiation was employed to introduce a tunable density of lattice defects, including sulfur vacancies. The evolution of surface chemistry upon ambient air exposure was then systematically monitored by XPS. High-resolution Mo 3d core-level spectra reveal a pronounced increase in oxidation rate with increasing ion fluence. Pristine monolayers exhibit minimal oxidation under ambient conditions, whereas irradiated samples show, in comparable timescales, progressive formation of higher oxidation states associated with Mo-O bonding. The oxidation kinetics are expected to follow a defect-density-dependent trend, consistent with vacancy-mediated oxygen adsorption. By correlating ion dose with the fraction of oxidized Mo species, we extract effective reaction rate constants and activation behavior as a function of defect concentration. The data support a model in which ion-induced sulfur vacancies act as primary nucleation centers for oxidation, lowering the kinetic barrier for oxygen chemisorption and accelerating lattice destabilization. At higher fluences, defect clustering further enhances reactivity, leading to spatially non-uniform oxidation and increased chemical heterogeneity. Importantly, the study distinguishes between direct beam-induced chemical modification and subsequent ambient-driven processes, demonstrating that ion irradiation alters reactivity. These findings provide insight into how controlled ion beam treatment modulates chemical reactivity in molybdenum disulfide. The results have implications for defect engineering and patterning strategies in MoS₂-based electronic and optoelectronic devices, where balancing functionalization and degradation remains critical.