Environmental barrier coatings (EBCs) are commonly used to protect SiC ceramic matrix composites (CMCs) from corrosion in combustion environments, thus extending their service life in hot-zones. The dominant process that affects the service life of CMCs is the thermal growth of oxide materials, which is facilitated by oxygen diffusion through the EBCs. In this study, the oxygen diffusion behavior through EBC materials, such as Sc2Si2O7, Yb2Si2O7, and Y2Si2O7, in a temperature range of 1100 °C–1300 °C was assessed using an 18O2 tracer. The 18O and 16O isotope exchange depth profiles were characterized by time-of-flight secondary-ion mass spectrometry (ToF-SIMS). The SIMS data were fit with a semi-infinite diffusion model to calculate the diffusion coefficients and activation energies for oxygen transport through the EBC materials. Y2Si2O7 had the lowest oxygen diffusion coefficient, while Yb2Si2O7 had the highest. The differences in oxygen diffusivity can be explained by the variations of the disilicate crystal structures in the different EBC materials. The analysis identified both crystal structure and bond length as dominant factors influencing oxygen diffusion. The methods and results of this study can be applied to the screening of EBC materials with strong oxidation resistance, thus ensuring a long service life for EBCs/CMCs.