The design of new polycrystalline Ni-based superalloys with enhanced oxidation resistance can enable higher operating temperatures in aeroengines with increased fuel efficiencies and minimised CO₂ emissions. This thesis investigated the oxidation behaviour in polycrystalline Ni-based superalloys with a combination of experimental and computational methods. The transient oxidation behaviour of the commercial alloy, RR1000, was investigated at 800°C with several characterisation techniques. The results showed a systematic evolution of oxide spectra with increasing oxidation times that could be related to the pseudo-linear kinetics. The sub-parabolic oxidation kinetics of RR1000 also differed significantly from similar studies. Wagner’s criteria for the internal-to-external oxidation transition for the formation and maintenance of a continuous Al₂O₃ scale were used in computational models to assess commercial alloys and an experimental alloy (Alloy X). The models successfully predicted the oxidation behaviour of the commercial alloys but underpredicted the transition temperature of Alloy X by 50-100°C, which may have been due to complex oxide formation. The model was used to design a pre-oxidation treatment for instigating an internal-to-external oxidation transition in a new Cr₂O₃-forming Ni-based superalloy (C19). The pre-oxidation treatment at 1100°C for 1 hour resulted in a continuous Al₂O₃ scale in C19, which dramatically reduced oxidation damage during further exposure at 800°C for 100 hours. The γ′ size evolution and topologically close-packed phase formation in C19 were also investigated and compared to RR1000. C19 exhibited insignificant coarsening at 700°C but showed more coarsening at 800°C after 1000 hours. The σ phase was also detected after 800°C and 1000 hours. The results suggested that C19 was less prone to coarsening than RR1000 but more work is needed to demonstrate its commercial viability. The oxidation performance of C19 at 800°C was further examined with systematic substitutions of Nb, Ta, and Ti for Al concentrations. The addition of Nb improved the oxidation resistance, which differed from the literature and may have promoted the formation of a CrTaO₄ layer as a diffusion barrier to oxygen ingress. The addition of Ta significantly improved the oxidation resistance and may have been due to the formation of AlTaO₄. The addition of Ti did not significantly affect the oxidation resistance, which may have been due to the relatively low Ti concentrations in the investigated alloys. Several of the samples with modified Ta and Ti concentrations also exhibited regions of continuous Al₂O₃ scale formation, suggesting that Type III oxidation behaviour could be induced in C19 with further refinement.